Refactor/make plotting optional dependency

[thangleiter]

Implement #25 and moreoever make the plotting.py module an optional extra.

[coveralls]

Pull Request Test Coverage Report for Build 225

• 75 of 76 (98.68%) changed or added relevant lines in 6 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-66.7%) to 28.224%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
filter_functions/util.py	8	9	88.89%

Files with Coverage Reduction	New Missed Lines	%
filter_functions/util.py	3	85.45%

Totals
Change from base Build 209:	-66.7%
Covered Lines:	192863
Relevant Lines:	590726

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💛 - Coveralls

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@0b42bfa). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #29   +/-   ##
=========================================
  Coverage          ?   93.72%           
=========================================
  Files             ?        7           
  Lines             ?     1866           
  Branches          ?      450           
=========================================
  Hits              ?     1749           
  Misses            ?       68           
  Partials          ?       49           

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