[WIP] Add secondary directory structure; add tests for conductivity

[htz1992213]

Summary

Add secondary directory structure; add tests for conductivity

• Added secondary directory structure for core and forcefield
• Added initial tests for conductivity

TODO (if any)

• Add more secondary directory structure
• Add more tests for conductivity

Checklist

Work-in-progress pull requests are encouraged. Please put [WIP]
in the pull request title.

Before a pull request can be merged, the following items must be checked:

• Code is in the standard Python style. The easiest way to handle this
  is to run the following in the correct sequence on your local machine. Start with running
  black on your new code. This will automatically reformat
  your code to PEP8 conventions and removes most issues. Then run
  pycodestyle, followed by
  flake8.
• Docstrings have been added in the Google docstring format.
  Run pydocstyle on your code.
• Type annotations are highly encouraged. Run mypy to type check your code.
• Tests have been added for any new functionality or bug fixes.
• All linting and tests pass.

[htz1992213]

Most of the targets have been achieved. Use separate PR to complement tests.