Needed updates for the regional map script.

[nusbaume]

New feature type

New plot and/or plot enhancement

What is this new feature?

The new regional mapping script still needs some additional functionality in order to match the abilities of the global contour maps. In particular two extra features are needed:

1. The ability to use observations instead of baseline model data.
2. The ability to skip plot generation if redo_plot is False.

Along with those changes, there will likely also need to be modifications to the website generator to handle the situation where instead of five seasons there is a list of variables for the same time/season. An example can be seen now if one uses the regional_map_multicase option with a large number of variables. The variable links show up, but they don't currently fit in the box/page.

Assistance required?

No, I will make a PR when the feature is ready

Extra info

Some of these modifications would also be a good, easy task for someone just starting out in ADF development.

[nusbaume]

Also need to add the ability to apply land/ocean masks, as implemented in PR #202 and discussed in issue #157

[nusbaume]

Also need the ability to apply scale factors and offsets in order to have input variables match the expected units, similar to what is done in the global lat/lon map plotting script.