Do not use values array in UserMetadata

[sandorkertesz]

With this PR:

• UserMetadata does not require "values" to be present in the input dictionary. Previously, under certain conditions, UserMetadata tried to access the "values" and it caused a crash when they were not available.
• It is possible to create a fully functional ArrayField with a UserMetadata without passing the values array shape to UserMetadata. E.g:

    vals = np.linspace(0, 1, 10)
    meta = UserMetadata(
        {
            "shortName": "test",
            "longName": "Test",
            "date": 20180801,
            "time": 300,
            "latitudes": np.linspace(-10.0, 10.0, 10),
            "longitudes": np.linspace(20.0, 40.0, 10),
        }
    )

    f = ArrayField(vals, meta)

    f.to_latlon() # this works

See the newly added test: tests/array_fieldlist/test_array_field_usermetadata.py

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.18%. Comparing base (8bc8c25) to head (00c6ecf).
⚠️ Report is 360 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #681      +/-   ##
===========================================
+ Coverage    90.15%   90.18%   +0.02%     
===========================================
  Files          159      160       +1     
  Lines        12181    12217      +36     
  Branches       595      595              
===========================================
+ Hits         10982    11018      +36     
  Misses        1017     1017              
  Partials       182      182              

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