Support sub-hourly plotting

[christinaholtNOAA]

The group would like to plot graphics at sub-hourly intervals.

The task will likely require some modification to the create_graphics.py driver, as well as how forecast hours are referenced in each of the relevant Classes. Likely a moderate Python lift.

A bigger lift is the ability to load such a large amount of data with the current memory-bound state we're in. This task may be better left for after memory-related issues of accumulating variables are addressed. Alternatives with the current state include fewer subdomains or variables to lighten the computational load.