Pivot to handle nexson 1 0 0

Changes here should handle Nexson 1.0.0 (as used in curation app). This is **untested** as I have trouble installing `ivy`. @rhr, please try these changes with different flavors of Nexson. To retrieve Nexson 1.0.0 using the phylesystem API, just add argument **output_nexml2json** to the query string like so:

http://api.opentreeoflife.org/phylesystem/v1/study/pg_2628?output_nexml2json=1.2.1