Start for a better data structure

[ptiede]

This is a staging ground for a better data interface. I will also likely include integration with VLBIData.jl

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 91.94%. Comparing base (82cafce) to head (9ed1e8f).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/observations/abstract.jl	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #418      +/-   ##
==========================================
- Coverage   92.29%   91.94%   -0.36%     
==========================================
  Files          33       33              
  Lines        1804     1812       +8     
==========================================
+ Hits         1665     1666       +1     
- Misses        139      146       +7     

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