Refactor for src style package

[Mark2000]

src directory style Python packages are preferred these days, so we should be using that. What are the specific benefits?

Considerations

• Make a clean break - sooner is better than later, but hopefully doesn't mess up branches in development (does handle this sort of refactor well). If waiting for the first release (alpha->beta) is better for this transition, lets do it then.
• Can we use a pyproj.toml like the cool kids do these days? Ensure it doesn’t break cluster install or how chebpy is installed.

[johnowagon]

Some benefits to this structure:

1. The main benefit is that src style is explicit, theres no ambiguity on what is available to import. Plus I think its cleaner.
2. As of PEP 518, using pyproject.toml is recommended anyways.
3. Not to mention configuring Sphinx will be much easier if this is done prior to setup.

From some initial research I don't think refactoring the structure is going to cause issues during rebases, however I haven't tested this yet, I'll jump on it tomorrow.

Side note, because the setup.py script ran some code along with installation, I had to specify an entrypoint in the pyproject.toml to achieve the same functionality. This changes the install command from pip install -e . to pip install -e . && finish-install.

[Mark2000]

Even though the install command would be longer/less simple, it'd probably better. I could imaging someone not liking that setup.py has calls to sys and downloads files from the internet with no warning; finish-install makes that more explicit.

We will want to test that pyproject.toml-based install works on the on the CURC cluster. I tried that with another small package I made for a class and couldn't get it to work due to the build-system specification, but didn't try very hard to find a fix to the issue. @johnowagon, @LorenzzoQM should be able to help you get an account on the cluster and get Basilisk installed there, he did it fairly recently and made some fixes to my broken cluster install script.