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