Guided Extrude Along Path

A path extrusion function that can be adapted for different functions. Two extrusion classes are currently implemented:

1. flat profiles in one axis
2. Vertically-aligned along a cylinder
3. Radially-aligned along a cylinder (may have difficulties with the end caps, as they're assumed to be a flat surface)

There is not yet a general path extrusion fall-back. If you're interested in that, try the 'Extrude Along Path' OpenSCAD code that inspired this change.