Craniotomies do not stay circular over the edge of the skull

[BlakeRussell98]

When moving the craniotomy over the edge of the skull, it loses its circular shape.
Also, the size of the craniotomy is adjusted using the 'r' variable. However, the size does not reflect the radius of the craniotomy but rather the diameter.

[dbirman]

Ah yeah -- they are being projected from above, when probably what we really want is for them to be projected perpendicular to the surface. Hmm -- I guess craniotomies could have angles which would fix this? Maybe I'll make it possible to set those through the API as a start. Let me think about this one!