Add support for ea_variable_codes in find_societies
Add support for ea_variable_codes in find_societies

With a list of EAVariableCodeDescription IDs, find_societies now
returns the societies coded with that value.

D-PLACE/dplace,stefelisabeth/dplace,shh-dlce/dplace,NESCent/dplace,stefelisabeth/dplace,shh-dlce/dplace,D-PLACE/dplace,NESCent/dplace,D-PLACE/dplace,NESCent/dplace,stefelisabeth/dplace,stefelisabeth/dplace,D-PLACE/dplace,shh-dlce/dplace,shh-dlce/dplace,NESCent/dplace