Add selector validator, more unit tests, rename several methods in plsel.py.
Add support for processing selectors with a single level, e.g., /foo (previously not done because of pandas pecularities).
Check index class correctly in make_index.
Make base.py consistent w/ renamed methods.
Fix regression in make_index, add more make_index tests.
Let [] be a valid input to make_index().
Add support for specifying port type in src_idx/dest_idx and make sure they return unique tuples in the order that they appear in the index.