Draft: Package reorganization and CI improvements

This MR moves model-specific pyDARM code into the model submodule and creates rules for the CI unit tests so that parts only run as-needed

With these changes, this still works:

>>> import pydarm
>>> foo = pydarm.sensing.SensingModel(...)

and this works

>>> from pydarm.model.sensing import SensingModel
>>> foo = SensingModel(...)

but this would not work

>>> from pydarm.sensing import SensingModel
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'pydarm.sensing'