Create an error if collapse_cell targets incorrect variable

Noticed that `imputer_race_alt.collapse_cell("inc_col == '>= 50% FPL' & famrace_col_alt == 'All black' & agegroup_col == '35 or older'")` triggered a 'successful' outcome when the wrong famrace variable was indexed. This might be something worth looking into. When variables input to the imputer differ from the variables defining cell definitions, this is possible... Not sure why.
