Support GPAW

[marco-2023]

We should consider to support GPAW *.gpw files.

These files use the ase.io.ulm format.

Methods for reading and writing this files are in
https://wiki.fysik.dtu.dk/ase/_modules/ase/io/ulm.html#Writer

[PaulWAyers]

@tovrstra do you have thoughts on this? I think there is some functionality already in denspart as I recall, which could (should?) be migrated over or copied to IOData I think.

[tovrstra]

Denspart does indeed have an interface to GPAW, which can be found here:

https://github.com/theochem/denspart/blob/main/src/denspart/adapters/gpaw.py

This interface does more than just reading data. It also imports GPAW to load the files and to compute the electron density on grids that are convenient for post-processing. Only a part of these features would fit well within IOData.