fixed cache dir bug

[Little-Ryugu]

Fixes #451 .

Changed cache_dir to default to str(pooch.os_cache("layup")) instead of None in LayupObservatory. Stops layup_furnish_spiceypy from searching for files in the Sorcha created cache.

Review Checklist for Source Code Changes

• Does pip install still work?
• Have you written a unit test for any new functions?
• Do all the units tests run successfully?
• Does Layup run successfully on a test set of input files/databases?
• Have you used black on the files you have updated to confirm python programming style guide enforcement?

[Little-Ryugu]

Don't accept/merge just yet. @mschwamb said there was a different bug before that caused a problem similar to this in orbitfit. I'm going to double-check everything first incase I missed something.

[Little-Ryugu]

@mschwamb, the fix was in pr #225 but missed the case where cache_dir is left as None. The call for Layup_observatory in orbitfit and predict was changed from:

Layup_observatory = LayupObservatory()

To:

Layup_observatory = LayupObservatory(cache_dir=cache_dir) 

This fixed the original bug in the main code, but the unit tests still have the bug as they call Layup_observatory= LayupObservatory()

There was another fix to a similar issue #286, but the fix was related to the observatory position and didn't change the unit tests calling the Sorcha cache.

[mschwamb]

I think you might want to make "layup" a global variable at the top of the file so it's clear that this is hardcoded - easier to spot if it needs to be changed.

[mschwamb]

see other comment about changes