Acquisition and maintenance expenses

Expense cashflow during the period from t to t+1. For any t, the maintenance expense is recognized, which is defined as:

pols_if(t) * expense_maint()/12 * inflation_factor(t)

At t=0 only, the acquisition expense, defined as expense_acq(), is recognized.

Changed in version 0.2.0: The maintenance expense is also recognized for t=0.