service to return set of trees included in the synthesis collections

[kcranston]

In order to complete the easy-add-tree-to-synthesis feature (see also this issue about nexson syntax), we need a service that will return the set of trees included in the collections used for synth. See MTH proposal in google doc for details.

[mtholder]

This does not cache the config for the synthesis or the list of trees, but https://devapi.opentreeoflife.org/phylesystem/v1/trees_in_synth now returns a collection that is a concatenation of the synth collections.

[mtholder]

Note that this is the set of trees queued for synthesis (not included in the last synth)

[jimallman]

Just for the record, we get the trees in last synth from /v3/tree_of_life/about (with {"include_source_list":true})