Input check helper functions

[kuriwaki]

Can have a set of functions that check if both popstrat and survey have the same levels and attributes, and are all shared by formula, etc..

[kuriwaki]

This is especially important given that ccesMRPprep::acscodes_age_sex_educ; ccesMRPprep::acscodes_sex_educ_race use different levels in educ (the former is four-way and the latter needs to use three-way education, so they need to have either an error check or variable rename) https://github.com/kuriwaki/ccesMRPprep/tree/fix_iss12