Memory leak fixes

[christinaholtNOAA]

This set of changes fixes a memory leak issue we'd been seeing that does not allow running subdomains on smaller Jet partitions (sjet/vjet and tjet/ujet). Summary of major changes:

• Removed a lot of caching as parallel mode doesn't really use that allocated memory anyway.
• Explicitly cleaned up larger memory objects.
• Clear and close images explicitly once saved.

These changes have been tested for HRRR and RRFS CONUS on a variety of jet partitions with desirable results, most notably the completion of subdomains, even several subdomains.

[christinaholtNOAA]

Excellent suggestion, @cshartsough. I will add a card to remind us to do that.

[christinaholtNOAA]

The linter checks for unused imports, so we should be covered there.

[Brian-Jamison]

Whoops, left my previous comment and just 'approved', sorry for the repeat.

Thanks for the clarification on the linter!