Configurable prefix

lets the creator of a Phlesystem instance specify the prefix for new IDs. These should probably be of the form: `[a-z][a-z]_` I can't recall if that is assumed somewhere, but the only tested form of ID is that form.
there is a new test in peyotl/test/test_phylesystem.py to verify that this works and is backward compatible to omitting the new_study_prefix arg.