Add support for filtering to a set of prefixes

[gaurav]

Filtering to a set of prefixes would allow searches to take the view of a single ontology or identifier source, allowing large and detailed identifier sources like UMLS to be filtered out when not needed. The specific need here is from the UI, who have found that filtering to MONDO (and possibly HP) removes a lot of problematic matches, and would like NameRes to handle this so that they don't need to reduce the number of results when they filter these results at their end.

Theoretically, there are two ways in which we could do this:

1. Filter only on the preferred identifier, i.e. concept A with a preferred ID of MONDO:1234 and a secondary ID of HP:4567 will be returned if filtering on MONDO but not if filtering on HP.
2. Filter on any identifier, i.e. concept A with a preferred ID of MONDO:1234 and a secondary ID of HP:4567 will be returned if filtering on MONDO AND if filtering on HP, but not on other prefixes.

I think the first approach is fine for an initial implementation, but we can also implement the second if needed. Of course a complete implementation would included both filter_only_prefix and filter_out_prefix fields, but to meet this use-case we only need filter_only_prefix.

[gaurav]

We have now implemented this for the first approach. Closing.