feat: Load csrs

[johnbdesanto]

This pull request adds functionality to read .pos files generated by the Canadian Spatial Reference System (CSRS) online PPP tool as input for GNSS positions.

[codecov]

Codecov Report

❌ Patch coverage is 97.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.80%. Comparing base (2b2e161) to head (1c1afe0).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gnatss/ops/io.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #404      +/-   ##
==========================================
+ Coverage   76.35%   76.80%   +0.44%     
==========================================
  Files          29       29              
  Lines        1793     1832      +39     
==========================================
+ Hits         1369     1407      +38     
- Misses        424      425       +1     

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