Autodetect dimension from units.

[anntzer]

This lets one write ScaleBar(1, "kpc") instead of the slightly redundant ScaleBar(1, "kpc", "astro-length").

Note that changes only occur in the constructor; "auto" is not when later calling set_dimension (directly or via the property) because that property's mutable semantics seem a bit fuzzy anyways: what happens if one sets the dimension to something incompatible with the units? I would believe ScaleBar.dimension should be a readonly property directly derived from ScaleBar.units.

Closes #70.

[codecov-commenter]

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Codecov Report

:x: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 91.93%. Comparing base (2c57173) to head (4b501ca). :warning: Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
matplotlib_scalebar/scalebar.py	85.71%	1 Missing :warning:
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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
+ Coverage   91.46%   91.93%   +0.46%     
==========================================
  Files           3        3              
  Lines         422      459      +37     
==========================================
+ Hits          386      422      +36     
- Misses         36       37       +1     

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