Fix contour plot error highlighted by ORCA2 grid example

Investigate and fix test failure whereby an IndexError can appear to prevent a contour plot being made from code which should work, specifically as per the (just refreeshed) docs example:

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ERROR: test_example_unstructured_orca_1 (__main__.UnnumberedExamplesTest.test_example_unstructured_orca_1)
Test example for unstructured grids: ORCA grid example 1.
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Traceback (most recent call last):
  File "/home/slb93/git-repos/cf-plot/cfplot/test/test_examples.py", line 1422, in test_example_unstructured_orca_1
    cfp.con(x=lons, y=lats, f=temp, ptype=1)
  File "/home/slb93/git-repos/cf-plot/cfplot/cfplot.py", line 3303, in con
    _cf_data_assign(f, colorbar_title, verbose=verbose)
  File "/home/slb93/git-repos/cf-plot/cfplot/cfplot.py", line 1379, in _cf_data_assign
    myz = find_z(f)
          ^^^^^^^^^
  File "/home/slb93/git-repos/cf-plot/cfplot/cfplot.py", line 10752, in find_z
    mycoords = find_dim_names(f)
               ^^^^^^^^^^^^^^^^^
  File "/home/slb93/git-repos/cf-plot/cfplot/cfplot.py", line 10724, in find_dim_names
    if field.coord(coords[i]).X:
                   ~~~~~~^^^
IndexError: list index out of range