Package structure review

[ray-chew]

@tbnc, what do you think of the refactored package structure?

Eventually, when the software packages mature, I will move them into separate repositories, e.g.:

1. pybella-common-utils
2. pybella-flow-solver
3. pybella-data-assimilation

Furthermore, while I will keep (and further move) functions with (to) numa-JIT, I believe that pyBELLA will benefit tremendously from having a JAX backend eventually.

[tbnc]

Neat!

Do I get it right that the .h5 files in the outputs folder are there for testing purposes? Normally those kinds of files would not be version controlled...

Perhaps we could have simply src rather than src/pybella, although this might change with the future repo restructuring.

Also, maybe the subfolders of src/pybella/interfaces are placeholders, but some of them contain only one file at the moment, so we could potentially reduce depth by one there?