feat diversion node

[jmccreight]

• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst or it's sub rsts?

[codecov]

Codecov Report

Attention: Patch coverage is 87.35632% with 11 lines in your changes missing coverage. Please review.

Project coverage is 71.93%. Comparing base (abd4b79) to head (b1e09ea).
Report is 43 commits behind head on develop.

Files with missing lines	Patch %	Lines
pywatershed/base/process.py	38.46%	8 Missing ⚠️
pywatershed/hydrology/source_sink_flow_node.py	95.38%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #327      +/-   ##
===========================================
+ Coverage    71.53%   71.93%   +0.40%     
===========================================
  Files           62       63       +1     
  Lines         8509     8635     +126     
===========================================
+ Hits          6087     6212     +125     
- Misses        2422     2423       +1     

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