[WIP] Organization of package contents so that `import testsystems` behaves as expected.

Ideally, we'd like `from testsystems import TestSystemName` to behave correctly from both *inside* the `openmm-testsystems/testsystems/` directory (for ease of debugging) and *outside* after installing via `setup.py` (for use).

I've started to work on this by renaming `test_systems.py -> testsystems.py` and creating an `__init__.py`, as well as editing the `setup.py`, but it isn't quite there yet.  Everything runs fine within the `openmm-testsystems/testsystems/` directory, but after installation, `import testsystems; help(testsystems)` doesn't list the available test systems in the docstring.