Accept path-like objects for path arguments (in addition to strings)

[ortk95]

All methods that take a path argument (e.g. Observation, Observation.save_observation, set_kernel_path) can now accept os.PathLike objects, in addition to strings. For example, these are now equivalent:

from pathlib import Path

Observation(Path('data.fits'))
Observation('data.fits')

This is mainly a change to type hints, with a few additional internal checks to ensure any path-like inputs are standardised internally to strings. Any path outputs/attributes (e.g. Observation.path) remain as strings.

Closes #348

Pull request checklist

• [x] Add a clear description of the change
• [x] Add any new tests needed
• [x] Run spell check on new text visible to user (documentation, GUI etc.)
• [x] Check any changes to requirements.txt are reflected in setup.py and conda-forge feedstock
• [x] Check code passes CI checks (run run_ci.sh or check GitHub Actions)

See CONTRIBUTING.md for more details.

[coveralls]

Pull Request Test Coverage Report for Build 8817888161

Details

• 9 of 9 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 99.574%

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Totals
Change from base Build 8724142762:	0.001%
Covered Lines:	2884
Relevant Lines:	2896

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💛 - Coveralls