feat(posfilter): roll-pitch-heading loader function should accept optional covariance rows

[madhavmk]

• Currently roll-pitch-heading loader function only accepts [time, roll, pitch, heading] rows. It should accept optional covariance rows "roll-roll", "roll-pitch", "roll-heading" ... .etc in the csv file as well.
• Add helper function gather_files_all_procs() that gathers file paths from all processes
• • Issue #222

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 58.33333% with 35 lines in your changes are missing coverage. Please review.

Project coverage is 50.44%. Comparing base (2f00e30) to head (b7e88e1). Report is 1 commits behind head on main.

Files	Patch %	Lines
src/gnatss/main.py	41.66%	14 Missing :warning:
src/gnatss/ops/posfilter.py	35.00%	13 Missing :warning:
src/gnatss/loaders.py	71.42%	4 Missing :warning:
src/gnatss/ops/data.py	40.00%	3 Missing :warning:
src/gnatss/cli.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #225 +/- ## ========================================== + Coverage 49.67% 50.44% +0.77% ========================================== Files 16 17 +1 Lines 930 1005 +75 ========================================== + Hits 462 507 +45 - Misses 468 498 +30 ```

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