A torus is represented with its location (point at center), axis vector of
the major spin, and the major and minor radii. The reference direction need
not be perpendicular to the axis vector but must not be parallel to it.
Parameterization is 0 to 2pi in u for the spin around the axis and -pi to +pi
in v along the (implied) generator circle. The 0 value for u and v is on the
outside of the torus (away from the axis). When looking at the torus from
the direction opposite the axis, -pi/2 of the generator is away from the
observer.

There are 3 classes of torus: doughnut, apple (including the oscillating
apple), and lemon. For a doughnut, major_rad > minor_rad > 0. For an apple,
major_rad <= minor_rad and major_rad > 0. The case where
major_rad = minor_rad is the oscillating apple and it is periodic in both
directions. For a lemon, major_rad < 0 and |major_rad| < minor_rad. These
may be determined by calling UF_MODL_ask_torus_face_type.