Enable converting fields with UserMetadata into Xarray

[sandorkertesz]

With this PR the examples below work:

data with geography

import earthkit.data as ekd

prototype = {
        "latitudes": [10.0, 0.0, -10.0],
        "longitudes": [20, 40.0],
        "values": [1, 2, 3, 4, 5, 6],
        "valid_datetime": "2018-08-01T09:00:00Z",
    }

d = [
        {"param": "t", "level": 500, **prototype},
        {"param": "t", "level": 850, **prototype},
        {"param": "u", "level": 500, **prototype},
        {"param": "u", "level": 850, **prototype},
    ]

ds = ekd.from_source("list-of-dicts", d)
a = ds.to_xarray()

data without geography

import earthkit.data as ekd

prototype = {
        "values": [1, 2, 3, 4, 5, 6],
        "valid_datetime": "2018-08-01T09:00:00Z",
    }

d = [
        {"param": "t", "level": 500, **prototype},
        {"param": "t", "level": 850, **prototype},
        {"param": "u", "level": 500, **prototype},
        {"param": "u", "level": 850, **prototype},
    ]

ds = ekd.from_source("list-of-dicts", d)
a = ds.to_xarray()

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.92%. Comparing base (f865858) to head (86e7595).
⚠️ Report is 334 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #701      +/-   ##
===========================================
+ Coverage    90.88%   90.92%   +0.03%     
===========================================
  Files          165      166       +1     
  Lines        12533    12586      +53     
  Branches       612      612              
===========================================
+ Hits         11391    11444      +53     
  Misses         960      960              
  Partials       182      182              

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