Update plot_maps.py for landice plotting

[trhille]

This merge provides several updates for the landice tool plot_maps.py:

1. It adds the ability for the user to define the range of values plotted using the new --vmin and --vmax arguments.
2. It adds reasonable bounds on velocities for plotting purposes.
3. It provides the ability to plot mesh variables that have no time dimension, such as gridSpacing, areaCell, etc.

[trhille]

Testing

Two fields, both with color ranges set automatically. bedTopography has a time dimension, gridSpacing does not. plot_maps -r $CFS/piscees/MALI_input_files/AIS_4to20km_r01/AIS_4to20km_r01_20220907_Bedmap2_surface.nc -v bedTopography,gridSpacing

Now manually setting color bar limits for gridSpacing, but allowing the code to automatically determine a reasonable range for bedTopography: plot_maps -r $CFS/piscees/MALI_input_files/AIS_4to20km_r01/AIS_4to20km_r01_20220907_Bedmap2_surface.nc -v bedTopography,gridSpacing --vmin None,0.e3 --vmax None,2.e4

Using internally calculated lower limit for speeds, while setting the upper limit manually: plot_maps -r $CFS/piscees/MALI_input_files/AIS_4to20km_r01/AIS_4to20km_r01_20220907_Bedmap2_surface.nc -v observedSpeed -l True --vmax 2.5

[xylar]

Don't worry about the failing tests. They're nothing to do with you. They're something I've tried to get fixed on conda-forge but I'm having trouble getting a response.

[trhille]

@matthewhoffman, thanks for the review! I addressed your requested changes in 02119d7.