Allow multiple transects within same CSET run

[jfrost-mo]

What problem does your feature request solve?

Currently we can only plot a single transect per model variable, with one set of start and end coordinates. It would be nice to plot multiple, to get a look at how they vary throughout the domain.

Describe the solution you'd like

Some kind of list of start and end points that we can then cycle over? Maybe something like the field definition?

Describe alternatives you've considered

Currently you can just run CSET multiple times to achieve this. It would be nice to do it in one shot.

[jwarner8]

An example use case is examining the winds through a channel. A user might want to create a transect across the front of the channel, through the middle and at the exit of the channel, to determine how the winds and moisture vary through the valley. We also need to ensure the proper metadata about the transect is included in the transect plot title (coordinate information).