Adding linking config variables

[astronomerritt]

Fixes #953.

• There is now a "hidden" config file variable, drop_unlinked, that controls whether unlinked objects are dropped by the linking filter or if they remain in the dataframe.
• By default, drop_unlinked=True.
• If linking is on and drop_unlinked=False then the tally file will contain the linking information. Otherwise it will not.
• Wrote unit tests to ensure expected behaviour in PPLinkingFilter() and PPStats().
• Updated unit tests.

Fixes #954.

• If drop_unlinked=False, SSP_linking_on=True and output_format=basic, the object_linked column now appears in the default basic output.
• Wrote a unit test to ensure the column is correctly included.

Fixes #975.

• A new config file variable, SSP_night_start_utc, controls the time in UTC at which the night "starts" at the observatory, which should be local noon.
• This config file variable is mandatory if linking is on.
• Changed all example/demo config files to include this variable with a default of 16.0.
• Updated unit tests.

NOTE: I used Chile Standard Time for Chilean noon, hence 4pm UTC. Mario was using Chile Summer Time, which is why he has 2pm Chile = 5pm UTC. I don't know which one is correct here so I defaulted to Chile Standard Time, will change it back if needed.

Review Checklist for Source Code Changes

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 64.70588% with 6 lines in your changes missing coverage. Please review.

Project coverage is 80.78%. Comparing base (9aae130) to head (cd20b7f). Report is 72 commits behind head on main.

Files with missing lines	Patch %	Lines
src/sorcha/modules/PPConfigParser.py	45.45%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #992 +/- ## ========================================== - Coverage 80.89% 80.78% -0.12% ========================================== Files 69 69 Lines 3094 3107 +13 ========================================== + Hits 2503 2510 +7 - Misses 591 597 +6 ```

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[mschwamb]

@astronomerritt can you resolve conflicts?

[astronomerritt]

Fixed!