Circular colorbar for azimuthal data

### Description of the desired feature

For various datasets a linear colorbar is appropriate. However when azimuthal data is plotted [0 - 360°] is would make sense to have a circular colorbar that actually represents the direction the data is plotted. This could either be set on the side of the plot or as an inset. At the moment I don't think this exists yet and my only work around would be to save a *.png figure and then add it to the pygmt plot through fig.image() function.

An example is the direction of tilt. It's possible to visualize this with vectors, but when the directions get more heterogeneous it gets messy. This is an example of where the direction of the ground tilt is visualized according to azimuth. The linear colorbar is confusing, as the colors are actually continuous and azimuthal.

the colorbar (colorcircle?) should be something like this:

### Are you willing to help implement and maintain this feature?

Yes