Allow clustersubspaces with no orbits

[lbluque]

Summary

Allows creating a ClusterSubspace with only a constant term (ie excluding pointer terms as well), by passing None as the point cutoff

subspace = ClusterSubspace.from_cutoffs(prim, {1: None})

Also slightly refactored private method _gen_orbits_from_cutoffs to call a _gen_point_orbits and _gen_multi_orbits for cleaner code and to avoid duplicated code in PottsSubspace

TODO (if any)

Needs a few test updates to simply check that it works and that the exception there is raised.

Checklist

• Code is in the standard Python style.
  Run pycodestyle and flake8
  on your local machine.
• Docstrings have been added in the Google docstring format.
  Run pydocstyle on your code.
• Tests have been added for any new functionality or bug fixes.
• All existing tests pass.

[lbluque]

merging. update tests with the bulk of updating to pytest.