Migrating trim and finders_table tests to unit_tests

I've moved `run_trim_test.py` and `test_build_finders_table.py` to `tests/unit_tests`. I've updated their import statments, and the import statements of some of the functions they call, to be of the form `mtpipeline.[foldername].[filename]`. 

I've moved the `ephem` folder from `MTpipeline/ephem` to `MTpipeline/mtpipeline/ephem`, so that it is built as a submodule of the `mtpipeline` module. I've also removed a duplicate `test_jpl2db.py` that was sitting in `ephem` (there is a `test_jpl2db.py` in the legacy tests, although its import statement needs correcting).

If this request is approved, we can close [Ticket#109](https://github.com/STScI-Citizen-Science/MTPipeline/issues/109).