[Bug] Richdem no longer available on PyPI for Python 3.8+

[gampnico]

Behaviour: installing COSIPY via pip with richdem fails for Python 3.8+.

Expected behaviour: COSIPY should install all its dependencies cleanly.

Possible solution: since richdemis now really outdated, I would suggest switching to xdem, which now has an established grounding within the glaciology community.

Some of the preprocessing scripts would need to be rewritten to support this.

[gampnico]

Note xdem requires Python <3.15, >=3.10, but we should probably update COSIPY's supported Python versions anyway.