Implement search by tipTaxa

Follows same logic as php site.Can specify 1 or 2 tipTaxa. If one taxon, it is looked up in the multi tree, and all ancestors are found. Calibrations associated with those ancestors are returned. If 2 tipTaxa supplied, the MRCA of the taxa is found, and the ancestry for all 3 nodes are used.

They are passed as array in a query string, e.g.

    curl "http://127.0.0.1:8081/api/v1/calibrations/?tipTaxa\[\]=aves&tipTaxa\[\]=dog"

Identified some issues with duplicates and empties, fixed those as well