PlasmaPy-data organization

[pheuer]

@namurphy @JaydenR2305 I think we need some organization for this repo! Two possible approaches

1. Mimic the PlasmaPy modules
   Pros: simple
   Cons: what if a file is used multiple places?

2. Group into broader categories
   Pros: Maybe clearer where to put files?
   Cons: We need to make up the categories

Possible categories:

• Tests (files for running tests)
• Documentation (files for notebooks or docs)
• Data (data files for functions in plasmapy, like the NIST data)