resolve issues with different ways of specifiying netcdf output optio…

[jmccreight]

…ns: in control.options vs model.initialize. solution is dont mix

• Closes Selected output variable netcdf files not writing to output folder #231
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst via update docs for release #257

[codecov-commenter]

Codecov Report

Merging #230 (3ff3be3) into develop (5dcbc04) will increase coverage by 0.06%.
The diff coverage is 95.65%.

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@@             Coverage Diff             @@
##           develop     #230      +/-   ##
===========================================
+ Coverage    79.55%   79.61%   +0.06%     
===========================================
  Files           51       51              
  Lines         6475     6491      +16     
===========================================
+ Hits          5151     5168      +17     
+ Misses        1324     1323       -1     

Files Changed	Coverage Δ
pywatershed/base/model.py	94.87% <95.65%> (+0.83%)	⬆️

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