Change ASE from submodule to pip

[elinscott]

Now that ase-koopmans can be installed via pip, we will migrate over to using pip to install it rather than having it as a submodule.

If you are a developer and you want to modify ASE, you must now...

1. clone ase-koopmans to a directory of your choosing
2. within this directory, run python3 -m pip install -e . to locally install ase
3. create branches, add commits etc.
4. create a pull request for ase-koopmans
5. request a review on the PR
6. once this PR is accepted and a new release of ase-koopmans has been created, update the version number of ase-koopmans in requirements/requirements.txt

[degennar]

Very nice! Switched to the installation mode and everything works fine! Thanks Edward!