Modified rvcData to subset data, added length_class, and when_present parameters

rvcData now primarily is for error checking and properly subsetting data. No aggregation of data takes place. That function has been taken over by the ssu method
Added ssu method to aggregate data by secondary sample unit
Modified psu to account for changes in rvcData and ssu methods.
Modified stratData to subset strata
It is now a helper method called within rvcData. TODO: Add weighting.
Modified strat to reflect changes to .psu

No longer takes a psu and ssu size argument. Takes only an rvcObj and calc argument
Modified stratData to calculate weighting for each stratum