Ability to produce hovmoellers

### What problem does your feature request solve?
For looking at tropical waves, we typically use hovmoellers (some spatial coordinate plotted as a 2D plot against time), to determine evolution and propagation as a function of time.

### Describe the solution you'd like
If we were to implement this, it would probably take 3 arguments (toggle, variable, vertical coordinate to use, level to extract, domain bound, including spatial coordinate to average.). It could leverage existing edits to the plot operator from transect so country lines are not plotted by default. Example's of hovmoellers can be found here https://www-nwp/~jawarner/oper/aew/plots/AEW_Hovmoeller.png.

### Describe alternatives you've considered
N/A - plotting outside toolbox.

Start by reviewing the existing transect plot operator, which the issue identifies as a possible foundation for hovmoeller plotting. Determine how to represent the proposed toggle, variable, vertical coordinate and level, domain bounds, spatial averaging, and default country-line behavior; done means producing the requested time-versus-spatial-coordinate plot for tropical-wave analysis.

Written by the indexing model from the issue text.