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

Correct development with only allowing memb_dim and time_dim to be different between data1 and data2; improved documentation; added unit tests

Improved development of MergeRefToExp; improved documentation; added unit tests for the new development

Develop SPEI in order that NA from accumulation are not used to compute standardization, then added again NA

Add in documentation that in result, the Dates correspond to the beginning of the aggregated time period

Correct parameter 'na.rm' in Standardization call, it was not included; corrected documentation for 'params'; added check for 'params' dimensions

Add time_bounds in functions that reduce time dimensions ; Add  element; Add checks if dates have dimensions; add tests for this development

Develop new element time_bounds with start and end elements containing the start and end dates of the aggregation for PeriodMean and PeriodAggregation; updated documentation; added tests

Add parameter dates_expcor; improve documentation; add warning for cross_validation; return correct values within 's2dv_cube'