Add support for delta printers with a radius option

[garethky]

In principal this should be straightforward to add. This isn't necessary for a delta to work, but it would be nice to have straightforward support.

It looks like rrf just defines a rectangular grid about the bed center and then dynamically clips all points in the grid that don't fall within that radius: grid.IsInRadius(x, y)

I still need to see how this is recorded in the heightmap.csv file. If you have a G29 probing file from a delta printer, please attach it to this issue.