consolidate Python fixtures

[stevekm]

Some unit tests use Python-generated fixtures such as demo mutations

These are currently scattered about in pluto-cwl, pluto, and helix_filters repos

Need to gather them all and keep them in more consolidated places (either per-repo or for all repos) so that its easier to find them

[stevekm]

This is started here; https://github.com/mskcc/pluto-cwl/blob/master/tests/fixtures.py

[stevekm]

Also here; https://github.com/mskcc/pluto-cwl/blob/master/tests/fixtures_fillout.py

[stevekm]

Also need to migrate the fixtures from pluto.settings to here as well; https://github.com/mskcc/pluto/blob/e10fd75b9f384b8d13ff84f6c955e08b2c354b4f/settings.py#L121

these data sets should be moved back into the pluto-cwl repo