Use @deprecated decorator for deprecated classes.

[JoerivanEngelen]

@Manangka came with this good suggestion in PR #1463 to mark deprecated functions/classes:

https://deprecated.readthedocs.io/en/latest/sphinx_deco.html#more-elaborate-example

But unfortunately when I tried it, I got a weird error message from sphinx autosummary:

sphinx.errors.ExtensionError: Handler <function process_generate_options at 0x00000178ADD19580> for event 'builder-inited' threw an exception (exception: no module named imod.evaluate)

But as far as I could tell, there normally is no issue with this module, the API docs for this module render fine.
As a builtin deprecated decorator will be included somewhere in Python >3.13, it is something interesting to pursue then.