Lots of improvements, reduction reserve and surrender value work; Excel export moved to separate file

- Move contract params (age, YOB, sum insured, etc.) to contract$params list
- Add absolute values for cash flows and present values (after premiums are
  calculated, the actual cash flows can be determined and valued)
- Excel export moved to separate file, no longer member function of the contract
- surrender value implemented, the function to calculate the surrender value
  can be supplied to the tarif upon creation (surrender values can be arbitrarily
  complex/non-standard, so a dedicated function passed to the tariff seems like
  the only way
- Implement alpha-cost refund (during the first 5 years) in two variations:
    o) linear refund (20% of the total zillmer sum per year)
    o) distribution to 5 years and then valuation as an annuity
- Implement calculation of the risk premium