interpr: fPeriodn -> fPeriods

Change-Id: I21b76d2d121b27c5b4aeb676948786ab91bff0b7
interpr: ZinsesZins is Compound Interest

There are many more, like ScZinsZ, ScLaufz, ScGetZw,...

And some I can't identify, because my financial vocabulary is limited:

nZr Zinsrate
nZzr Zinseszinsrate
nBw ?
nZw Zinswert
nRmz ?

and many many more ;)

Change-Id: I11c26a8d4519bbd1e8242d27d3815db2bc3fdecd