Enable plotting of cfdm fields with cfplot

[sadielbartholomew]

In recent discussions relating to the CF Training course, we agreed it would be useful to be able to plot cfdm fields, and not just cf-python fields, with cfplot. Some work would probably need to be done on both the cfdm and cfplot side to make this possible.

For example, with the latest versions of all of the libraries, taking a simple example (lifted from the documentation and using one of the sample data sets) with trying to plot a field read-in via cf and cfdm separately, the cf field plots fine but the cfdm one runs into some cfplot errors due to shape:

>>> import cfplot as cfp
>>> import cf
>>> import cfdm

>>> cf.__version__
'3.11.0'
>>> cfdm.__version__
'1.9.0.1'
>>> cfp.__version__
'3.1.17'

>>> cf_f = cf.read('tas_A1.nc')[0]
>>> cfdm_f = cfdm.read('tas_A1.nc')[0]

>>> cfp.con(cf_f[0,0::])  # plots successfully
>>> cfp.con(cfdm_f[0,0::])  # errors
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/sadie/anaconda3/envs/cf-env/lib/python3.8/site-packages/cfplot/cfplot.py", line 401, in con
    check_data(field, x, y)
  File "/home/sadie/anaconda3/envs/cf-env/lib/python3.8/site-packages/cfplot/cfplot.py", line 3779, in check_data
    raise Warning(errstr)
Warning: 
Input arguments incorrectly shaped:
x has shape:(73,)
y has shape:(1,)
field has shape(1, 73, 96)

Expected x=xpts, y=ypts, field=(ypts,xpts)
x=npts, y=npts, field=npts
or x=[ypts, xpts], y=[ypts, xpts], field=[ypts, xpts]