Fix paths to saved sub-catalogs

[duckontheweb]

Related Issue(s):

• Closes Incorrect path to saved sub-catalogs #713

Description:

Fixes the paths to which sub-catalogs are saved by ensuring that the dest_href argument to Catalog.save is a directory when called on child catalogs.

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Merging #714 (a0c3e41) into main (70faaa5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #714   +/-   ##
=======================================
  Coverage   94.30%   94.31%           
=======================================
  Files          77       77           
  Lines       11249    11268   +19     
  Branches     1342     1344    +2     
=======================================
+ Hits        10608    10627   +19     
  Misses        461      461           
  Partials      180      180           

Impacted Files	Coverage Δ
pystac/catalog.py	92.13% <100.00%> (ø)
tests/test_catalog.py	98.96% <100.00%> (+0.02%)	⬆️

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[duckontheweb]

cc: @KennSmithDS