Better control of ScaleBar thickness.

[anntzer]

Closes #69. There is a slight backwards incompatibility regarding the rcParams (I just brutally replace the scalebar.width_fraction entry by scalebar.thickness), which is rather annoying to make fully backcompatible. But I can try to improve on that if that's something you care about and if you agree about the rest of the general design.

Open questions:

• Should width_fraction be deprecated?
• Should length_fraction get the same treatment? If so, should fixed_value/fixed_units be rolled into length, e.g. as length=(1, "um")?

[codecov-commenter]

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Codecov Report

:x: Patch coverage is 81.57895% with 7 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 89.35%. Comparing base (2c57173) to head (a319330). :warning: Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
matplotlib_scalebar/scalebar.py	81.57%	7 Missing :warning:
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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
- Coverage   91.46%   89.35%   -2.12%     
==========================================
  Files           3        3              
  Lines         422      479      +57     
==========================================
+ Hits          386      428      +42     
- Misses         36       51      +15     

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[anntzer]

@ppinard Kindly bumping?