Improved handling of coordinate variables

[iguinn]

• Handle binary operators on coordinate variables correctly. Before, result of binary operator just inherited the first variables coordinate grid and status as a coordinate variable. Now, it:
  • If both variables are coordinates, treats the result as not a coordinate. This is because the most likely use case for this is for differences between time points, and these should not be coordinates
  • If one variable is a coordinate, inherit the values from that variable. This is because the most likely result for this is applying an offset to a coordinate (e.g. 5*us from tp_0)
  • If neither variable is a coordinate, result is not a coordinate
  • If these cases are not suitable, user will have to specify it
• Fixed failure to convert units correctly for coordinates when writing output buffers for VectorOfVectors and ArrayOfEqualSizeArrays
• Made single-value subscripts work as expected
• Fixed error raised when adding constant values
• Fixed bug that prevented all values in VoV and AoESA to be drawn by waveform browser
• Constant values are by default not treated as coordinates
• Other small bug fixes and code cleanups

[codecov]

Codecov Report

Attention: Patch coverage is 48.14815% with 28 lines in your changes are missing coverage. Please review.

Project coverage is 61.87%. Comparing base (758c7c9) to head (74dfe76).
Report is 16 commits behind head on main.

Files	Patch %	Lines
src/dspeed/processing_chain.py	47.05%	27 Missing ⚠️
src/dspeed/vis/waveform_browser.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #63      +/-   ##
==========================================
+ Coverage   61.34%   61.87%   +0.53%     
==========================================
  Files          46       46              
  Lines        3073     3111      +38     
==========================================
+ Hits         1885     1925      +40     
+ Misses       1188     1186       -2     

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