Support export of NURBS as NURBS via OBJ format

[virtualritz]

Apparently NURBS Python converts NURBS geometry to triangle meshes on export to OBJ. This implicit conversion is nowhere mentioned in the documentation. There should be a big warning box added.

Wavefront OBJ supports (trimmed) NURBS geometry. See e.g. a copy of the format description at Paul Bourke's website.

Software that can read (and write) such OBJ files containing NURBS data are e.g. Rhinoceros or Maya.