Add pwmantel for pairwise mantel tests

Core algorithm is working. No tests, useful error messages, or coaxing of distance matrix IDs to a compatible order. p-values need to be formatted for the correct number of digits. Still not sure whether to return a DataFrame or some sort of results object that stores a DataFrame with extra metadata. Going with DataFrame for now as it's simpler.
Update test_sort.py

deprecated assertEquals changed to assertEqual
Update test_factory.py
Update test_factory.py
Update test_factory.py
Update test_Factory.py
Port make_compatible_distance_matrices; add basic tests
Update test_factory.py
Update test_sort.py
using with to open files to avoid ResourceWarning
Format p-values as strings w/ correct num decimals
Update test_factory.py
Update factory_py
Update factory.py
Add tests for N/A p-values and too few permutations
Update skbio/parse/sequences/factory.py
_factory.py
Add DissimilarityMatrix.index method
Add DissimilarityMatrix.__contains__
Add DissimilarityMatrix.filter