Add string-formatted amortization and first payment date of next term

Change MortgageCalculatorResult to return:
- String: RemainingAmortization e.g. "20 years, 0 months"
  This is used to give a human-readable representation
- DateTime: FirstPaymentNextTerm
  This is the default first payment date of the next term which can be
  used to automatically advance (assuming renewal under same terms)