Add fold composition feature to the continental plate

[MFraters]

Add a way to easily add fold (anticline/syncline) as compositional plugin to the continental plate feature. It is currently kind of possible through the layers feature, but is uses a linear interpolation and would be hard to add. Having a composition plugin which would allow you to quickly add a multiple layer fault would be useful.

Source: https://commons.wikimedia.org/wiki/File:Anticline.png#/media/File:Anticline.png

source: https://commons.wikimedia.org/wiki/File:Flank_%26_hinge.PNG#/media/File:Flank_&_hinge.PNG