Adds flatten argument to to_numpy methods

[JamesVarndell]

Adds the flatten argument to xarray DataArray and Dataset wrapper classes:

• If flatten=True, reshape the resulting NumPy array to be 1-dimensional after converting with to_numpy().
• If flatten=False (default), maintain the current behavior and return the regular array.

See #684 for more information.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.86%. Comparing base (006632f) to head (f5a80db).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #685   +/-   ##
========================================
  Coverage    90.85%   90.86%           
========================================
  Files          160      160           
  Lines        12136    12145    +9     
  Branches       593      593           
========================================
+ Hits         11026    11035    +9     
  Misses         930      930           
  Partials       180      180           

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[sandorkertesz]

@JamesVarndell, thank you very much for this development. Please can I ask you to add a test to tests/wrappers/test_xarray.py?