Store selected variable code objects in searchParams

Not just object IDs
Simplify min/max calculation

Gave me an excuse to use Math and apply
Store selected geographic region objects in searchParams

Not just object IDs
Store selected language classification objects in searchParams

Not just language ids
Change variable name from filter to environmental_filter

Avoid ambiguity with builtin 'filter' function
Fixing tests

Now that API views expect full objects, tests need to send full objects.
Still lots of tests to fix
Finish cleaning up tests for find_societies

All objects POSTed to API are now full objects, not just ids
Fix typo in test_find_society_by_language_and_var
Merge pull request #181 from NESCent/keep-trait-codes

Keep trait codes