Improve documentation of PeriodStandardization, PeriodAccumulation, PeriodSPEI and PeriodPET; correct few errors in the code

Update with parameter 'params' function PeriodStandardization and update PeriodSPEI with it; updated unit test and documentation

Correct function PeriodAccumulation with rollingaccumulation; corrected unit tests, updated PeriodSPEI; corrected PeriodStandardization

Add new function PeriodStandardization; change variable names in PeriodPET and improve PeriodAccumulation