Feature list for plotting script

[mkavulich]

There are a lot of features we would like to add to the MPAS plotting script, so this issue will track those

To do

• Add the capability to query the available variables, vertical levels, etc. This may make sense as a separate script rather than within the plotting script
• Option to coastlines and political boundaries
• Option to specify map projection
• Support for edge- and node-centered variables (only face-centered variables work for now)
• Add parallelism. Current serial output is very slow for large MPAS files due to the need to convert from a 1-d connected grid to a 2-d regular grid (for example, running on just our coarse global grid takes several seconds, and experimental HAFS output at 3km took more than a minute per plot). With not too much work it should be possible to add parallelism to this script (for example, the way that pygraf does it