Make argument names consistent, update docstring, & add units

[FarnazH]

• Update the docstring of the argument center and specify atomic units
• Arguments sectors_degree and deg_sectors were used to refer to degree sectors. so they were both changed to d_sectors.
• sectors_r and radius_sectors were used to refer to radius sectors, so they were both changed to r_sectors. Unit was clarified in docstring.

[marco-2023]

The changes are good, but now several tests are failing (I will update them).

[marco-2023]

@FarnazH I had to use defaults (None) for the degree and size arguments of the AngularGrid __init__. If no default argument is given for degree, then, when initializing from size, it is necessary to provide a degree=None. This was breaking many of the tests. Please feel free to change this (it just was the only way that I could find to solve this issue).

[FarnazH]

@FarnazH I had to use defaults (None) for the degree and size arguments of the AngularGrid __init__. If no default argument is given for degree, then, when initializing from size, it is necessary to provide a degree=None. This was breaking many of the tests. Please feel free to change this (it just was the only way that I could find to solve this issue).

@marco-2023, the degree should be set to None by the user if they want to make a grid using the size. That's the API change I proposed in PR #239, which is merged. In my opinion, it does not make sense to set the default values of None for degree and size when at least one of them should be provided. I understand that we want to support two ways of initializing the grid, but the concept of optional arguments is misused. As it is more common to use degree, the user can set it to None, if they choose to use size.

We can talk more about this today, as this is an important API change appearing in other classes as well.

[FarnazH]

@Ali-Tehrani and @marco-2023, the notebook failure can be ignored for now. We are changing the API, and just updating the JCP_paper.ipynb that is needed for submitting the paper. Then we can go back and fix the other tutorial notebooks.

[Ali-Tehrani]

• The changes made here don't work for python 3.7, so I added the fixes in commit afb9a8e. Also added python 3.7, 3.8 to github actions to make it more evident (I thought I did this before but guess not).
• Fix the ruff linter complaints with using Union[a, b] to a | b.
• Fixed the importlib problem with using Python =3.7.

I'm indifferent on either API choices. As long as it's documented, I'm okay with either.

[Ali-Tehrani]

My bad , I didn't realize running JCP_paper was installing the master branch of grid and so I was constantly getting errors since the changes in the Pull-request weren't made. I think this is good to merge and will fix most of the problems.