Prevent removing only common subsumers in resnik similarity

[johnbradley]

The resnik_similarity() function removes terms with 0 frequency. It is possible that this logic might remove the only common subsumer between two terms which would produce invalid results. Prevent this problem from occurring.

See #235 (comment) for more details.

[johnbradley]

@hlapp suggested the following:
After removing terms with 0 frequency check Jaccard similarity. If any terms have a Jaccard similarity of 0 raise an error instead. It would also be good to show a warning when removing any rows as users might not expect that this is happening.

[hlapp]

After removing them, not before.