Implement various amortization types for term loans

[bosue]

Term loans are not necessarily linearly amortized to 0.

There may be a residual value in the end, and the amortization may be progressive or (more often) degressive rather than linear. There doesn‘t even have to be any amortization, just payment of interest or not even that, and still it is a term loan, just not an amortized one.

See the list of amortization types as proposed in #54.

While we want to strip down #54 to a minimum viable product, this may be implemented independently, either right away or later. For reasons of complexity it‘s probably gonna happen later though.