Add subsetting option to data access methods

[sandorkertesz]

Fixes #406

This PR adds the index option to the following methods on a field/fieldlist:

• to_numpy()
• to_array()
• data()
• to_latlon()
• to_points()

With this the extracted values can be sliced on a per field level. E.g. if ds is a fieldlist we can write:

v = ds.to_numpy(flatten=True, index=slice(0, 3))

This is roughly equivalent to:

v = np.array([f.to_numpy(flatten=True)[:3] for f in ds])

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.06%. Comparing base (4973af9) to head (99e47cb).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #407      +/-   ##
===========================================
+ Coverage    89.92%   90.06%   +0.14%     
===========================================
  Files          120      120              
  Lines         7764     7875     +111     
  Branches       675      675              
===========================================
+ Hits          6982     7093     +111     
  Misses         651      651              
  Partials       131      131              

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[sandorkertesz]

@iainrussell, if it is fine with you I merge and close this PR

[iainrussell]

Looks good to me!