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

Develop SelectPeriodOnData and SelectPeriodOnDates to work with the new s2dv_cube and impose Dates to have always dimensions asigned