Figure out why diff plots are running so slowly

[pinarda]

Get a python profiler running to check this

[pinarda]

cProfile.run("ldcpy.plot(ds, 'TS', c0='orig', c1='zfpA1.0', metric='std', metric_type='diff')")

relevant output:

234978889 function calls (234151941 primitive calls) in 197.699 seconds

Ordered by: standard name

ncalls tottime percall cumtime percall filename:lineno(function)
.....
1 0.000 0.000 195.859 195.859 geoaxes.py:1487(contourf)
.....

SciTools/cartopy#1291

This appears to be an issue with resizing the scale limits inside contourf. This makes sense, because the only difference between the contourf calls in a regular spatial plot and the diff spatial plot is that most of the values are near to zero in a diff plot. The simplest solution, suggested elsewhere online (https://groups.google.com/forum/#!topic/scitools-iris/kuO2cnafrF0), is simply to use pcolormesh instead (issue #53).