Resolve "Clean up `src/fgen/templates`"

Description

• fixed up naming throughout the directory
• made Fortran and python structures more symmetric so it is easier to see what is going on and how things map to each other
• docs explaining how these templates are set up

Checklist

Please confirm that this pull request has done the following:

- [ ] Tests added

• Documentation added (where applicable)
• Changelog item added to changelog/

Closes #55 (closed)