Use importlib to track data files

[ljlamarche]

Description

This PR transitions to using importlib.resources to keep track of data files instead of paths relative to the source file. This is the new recommended way of managing package data, as shown in this description of packaging data and this example.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that changes existing functionality) - seems to break some unit tests; this is a work in progress

How Has This Been Tested?

Running unit tests - not fully functional yet.

Test Configuration

• Operating system: MacOS 12.6.1
• Python version number: 3.10
• Compiler with version number: gfortran 12.2.0
• Relevant local setup details: apexpy 2.0.0

Checklist

• Make sure you are merging into the develop (not main) branch
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• Add a note to Changelog.rst, summarising the changes
• Add yourself to AUTHORS.rst and .zenodo.json

[aburrell]

Just a few style things to change. Otherwise this looks like a great improvement. Kinda shocked it fixed some of the OS X failures 🐞