Fixed bug in handling of coordinates

[iguinn]

See #18

• Added unit test to help catch cases like this
• Added fix to bug in handling of units
• Added fix to addition of quantities to coordinates (e.g. 5*us + t0) not working if quantity comes first
• Added fix to slices adding 0 unnecessarily

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (e3dacf2) 61.56% compared to head (c31239f) 61.69%.

Files	Patch %	Lines
src/dspeed/processing_chain.py	83.33%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
+ Coverage   61.56%   61.69%   +0.13%     
==========================================
  Files          40       40              
  Lines        2677     2684       +7     
==========================================
+ Hits         1648     1656       +8     
+ Misses       1029     1028       -1     

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