MAINT: Move `CastepParam` and `CastepCell` into the `io` directory

My goal is to move the Castep.read contents in read_castep_castep. For this aim, to avoid cumbersome circular import treatment, in the present MR I would like to first decouple CastepParam and CastepCell from ase/calculators/castep.py and move them to the io directory.

While the changes are seemingly drastic, what I did is essentially just moving the contents from calculators to io and just adjusting import statements. The present MR passes the pipelines.

To also avoid further lengthy files, I also renamed ase/io/castep.py to ase/io/castep/__init__.py and put CastepParam and CastepCell in a new file ase/io/castep/castep_input_file.py.

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Issue is resolved via "closes #XXXX" if applicable.