Koopmans

This is release v0.4.0 of `koopmans`.

This is a minor release that makes several substantial changes to the workflow code base with very few consequences for the user.

Most notably for the user...
- `koopmans` now requires python v3.7 and above
- there are a few fields in the input file that have changed format (the kpoints and the wannier90 projectors)

Other changes include...
- The `Calculator` classes are now child classes of the corresponding ASE calculators rather than having an `_ase_calc` attribute
- The `Workflow` class has been reorganised
- settings are now managed using the `SettingsDict` class, a smart class that behaves like a dictionary but with extra checks and balances
- more extensive use of typechecking and `pathlib`

This version of the code now includes the python-based workflows of [python_KI](https://github.com/elinscott/python_KI).

From this point onwards updates to the python workflow will be made directly to this repository. [python_KI](https://github.com/elinscott/python_KI) will not be maintained and will exist for legacy purposes only.

This was the version of the code used for the calculations in Riccardo's paper. It contains:

- `cp_koopmans`: the Koopmans CP code
- `qe_koopmans`: a fork to the official Quantum Espresso GitLab repository (v6.7) containing the modifications to `pw2wannier90` and the `KC_WANN` implementation
- `unfold_and_interpolate`: the old version of the unfolding Python code. At this stage this was already implemented within `python_KI` and the version present was not used anymore.

The calculations present in the paper were made using the version [4425d4c](https://github.com/elinscott/python_KI/commit/4425d4c22eeed036d427c7c99aeeb351ed217d62) of the [python_KI](https://github.com/elinscott/python_KI) repository.

Old version of the code before forking the official `QE` repository and cleaning the local copy of the `CP4.1` code.