Check that generate_test_plots.py still works even when coords have infs / NaNs (e.g. when using the full geo extent of the RSS imagery)

[JackKelly]

Detailed Description

The code doesn't mask inf or NaN coordinates. Need to make sure that generate_test_plots.py still works when plotting full geo extent of the RSS imagery

[jacobbieker]

It works for geostationary when plotting with those coordinates, not necessarily with OSGB coordinates for the full extant though

[jacobbieker]

It does with the geostationary plots, so closing this for now, especially as we are moving away from OSGB coordinates