Cubic map projection

[AdamRJensen]

Description

I have a need to utilize a cubic map projection (which as an example is supported by ArcGIS and NASA's G.Projector has a projection called gmomic cubed square).

Would this be a desired feature to have added to Cartopy? If so, I'd be willing to work on it but would need significant assistance as I've not worked with map projections before.

Perhaps the code in this example could be utilized?

[rcomer]

There was a prototype of something like that at #1248.

[AdamRJensen]

The code posted in #882 seems to work nicely.