Study file

deployed on ot10.

depends on https://github.com/OpenTreeOfLife/peyotl/pull/22 being merged to master

Extends https://github.com/OpenTreeOfLife/phylesystem-api/pull/109 (so don't merge this before that one is merged).

Adds support for study/ID/file and study/ID/file/ID methods for retrieving supplementary files.

documented in docs/README.md edits.

New test in ws-tests/test_file_get.sh