-) Premium, commission and deferral periods must be
   at most equal to the contract period => add check
   and use min to cut off

-) Add commissionPeriod parameter
-) Add CommissionPeriod cost duration dimension (i.e. add one value to the corresponding dimension of the costs array)
-) Add alphaCostsCommission parameter to indicate how alpha costs over the commission period are to be understood: actual / linear / presentvalue

-) Add adjustPresentValues(Costs) hook functions to allow modifying present values

-) Add benefitParameter parameter to pass contract-/tariff-specific information to custom functions

For endowments, sumInsured shall be constant over the whole contract, even though the death benefit might be only a fraction of the sumInsured

The actual death benefit ist stored in a time series cf$death, so cf$sumInsured shall represent the real, contractual sumInsured over the whole contract period.

Introduce hooks Hooks$premiumRebateCalculation and Hooks$adjustPVForReserves to modify the premium rebate calculation (e.g. a rebate relative to admin cost, or admin cost reserves that exclude a premium rebate).

The premiumRebateCalculation hook is called after the premiumRebate rate is evaluated and can be used to adjust the rate to some other value that e.g. includes reserves / future present values, etc. This way, one can have a varying rate, but always the same logic to apply the rebate rate. So parameter and calculation are separated cleanly.

The adjustPVForReserves hook is called before all present values are combined to the corresponding reserves. This allows modifying all present values that will be used in the reserves. E.g. one can reduce the admin cost present values by a premium rebate and the admin cost reserve properly discounts the rebate from the reserve. The PV modifications have NO influence on the actual present values stored for the contract or used for other purposes. Only the local copy of the present values in the reserve calculation funciton is modified.

When a single-premium contract is used to emulate a paid-up contract with premium refund, the past premiums paid need to be given externally and is not related to the premium calculated for the contractis relative to the (new) sum insured.

These unit costs should no longer be added after the gross premium

Add attributes Parameter, which can be used by tariffs to enable contract-specific behavior (e.g. an indicator whether a contract is the main slice or a top-up / additional payment with different costs, etc.).

Access via contract$Parameters$ContractData$attributes$...

-) Params deathBEnefitProportion and widowBenefitProportion haven't been used => removed

Add hooks (adjustCosts, adjustMinCosts) to allow modifying costs per contract (i.e. modifying existing cost definitions from the tariff); Add function costs.scsaleAlpha

Allow (premium|benefit)FrequencyLoading to be a function or a fixed value; set premiumFrequencyOrder to -1 if loading is given

The (premium|benefit)FrequencyLoading parameters are evaluated using a new function evaluateFrequencyLoading, which handles functions, named lists, and fixed values.

Also, set premiumFrequencyOrder automatically to -1 (i.e. no adjustment due to multiple payments per yer) when a loading is given.

-) Add parameter birthDate, deprecate YOB instead (but fall back to YOB)
-) Add functions age.exactRounded and age.yearDifference for the two most common age calculation methods (nearest birthday at contract closing or difference of calendar years)

By default, the gross premium does not include unit costs, which are added after gross premium (but before taxes). Some companies include the unitcosts in the gross premium (which is refunded upon death -> correct calculation is essentaion). This flag allows a tarif to include the unit cost in the gross premium reather than adding later.

Fixes Issue #73 (except for the documentation, which is covered by a different issue anyway)

Fixes #71

vmGlgExamples: Fix errors due to switch from vector to list (subsetting generates NULL entries rather than skipping them altogether)

ifelse cannot return NULL, so we need to use if (has.prf) {...}; The (missing) else will silently return NULL, so the value will not show up in the list of values.

-) Add costWaiver and minCosts params to define base costs (per tariff) and the percentage of the waiver (per contract)
-) Add helper function costs.baseAlpha to define a base alpha cost while leaving all other cost items unwaivable

Implements the code-parts of issue #64

Add parameter useUnearnedPremiums to switch between unearned premiums and inclusion of full (single) premium in balance sheet reserve

Single-premium contract typically do not use unearned premiums, but add the whole single-premium to the balance sheet reserve immediately. The parameter useUnearnedPremiums switches this behaviour.

By default, single-premium contracts will not use unearned premiums (and include the full premium in the balance sheet reserve), while contracts with regulare premiums will use unearned premiums by default (and not include the premium paid at time t in the balance sheet reserve at time t).

Closes #69

For the balance sheet reserves we might need the net/gamma/Zillmer premium components (for single-premium contracts no unearned premiums are reported, but the premium is included in the reserve!), so we need to calculate it before.