Multiple values when querying a taxid or a obtid

Parameters obtid, taxid and docid only accept a single value.

Calls to the API could be reduced if these parameters accept several values.

Example: /search/relations?taxid=ncbi:1423&taxid=ncbi:521

would search the set of relations that is the union of relations for ncbi:1423 and those of ncbi:521. For each result, taxroot would indicate which of the two roots is matched.