Add SpectralUnitsWarning for more targeted warning filters

[lpsinger]

This makes it easier for users to disable the unit conversion warnings without also suppressing other unrelated UserWarnings.

This should be a backward-compatible change: since the new warning class subclasses UserWarning, code designed to suppress UserWarnings will still suppress this warning.

[karllark]

Please add a description to this PR to describe in words the goal. Thanks.

[lpsinger]

Please add a description to this PR to describe in words the goal. Thanks.

Done.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.89%. Comparing base (8871ed6) to head (412cbfc). Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #239   +/-   ##
=======================================
  Coverage   99.89%   99.89%           
=======================================
  Files           8        9    +1     
  Lines         935      938    +3     
=======================================
+ Hits          934      937    +3     
  Misses          1        1           

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