Add new features to declarative syntax for gridded datasets

[kgoebber]

There are a couple of small things that could be added to the declarative syntax to allow an even more robust experience for users for gridded datasets...

• scale variables (e.g., vorticity)
• specify colors for contours/filled contours instead of using cmap
• multiple titles (e.g., left and right side)
• convert units

[jthielen]

• scale variables (e.g., vorticity)

If I'm interpreting this right (that this is #3), I'm not sure that this is a small thing, but it would be great to have! After xarray and unit integration is improved with v1.0, and the various CF-metadata improvements are added (#886 and the generalized versions of the WRF metadata helpers I still haven't gotten done yet), I'd like to try taking this on some time in the spring.

[kgoebber]

Yes, to some extent it is similar to #3, but even if we had that, there would still be a need to scale. For example, vorticity is on the order of 10^-5, so a scale of 5 could be set to multiply the variable (whether native to the file or computed) by 10^{scale} to plot values that are more sensible.

[dopplershift]

Still some things left to do here...