CRTOSU ( centre, axis, majrad, minrad, torus, ifail )
=====================================================

  Create a toroidal surface.

Receives:
  KI_vec_centre             centre         --- centre position
  KI_vec_axis               axis           --- axis direction
  double                   *majrad         --- annular radius
  KI_dbl_radius            *minrad         --- radius of generating circle

Returns:
  KI_tag_surface           *torus          --- toroidal surface
  KI_cod_error             *ifail          --- failure code

Specific errors:
  KI_radius_sum_le_0        majrad < zero and majrad + minrad <= zero
  KI_radius_too_large       invalid radius value
  KI_radius_eq_0            invalid radius value

Description:
  A complete toroidal surface is created with centre 'centre', defined by
  rotating the generating circle (of radius 'minrad') about 'axis'. The major
  radius 'majrad' (radius of the spine circle) is the distance from 'centre'
  to the centre of the generating circle.

  The surface normal at any given position points away from the nearest
  point on the spine circle.

  The locus of the torus is a closed surface sheet. However for certain
  choices of 'majrad' and 'minrad' this surface intersects itself creating
  two opposing singular points where the surface normal is undefined. The
  outer sheet of self intersection is called an apple. The inner sheet of
  self intersection is called a lemon. Each is bounded by the two
  singular points and treated as a separate surface.

  This routine creates one of the three variations of a torus; non-self
  intersecting surface, lemon, or apple depending upon the values of
  'majrad' and 'minrad'. If 'majrad' is positive and smaller than 'minrad'
  this defines an apple. If 'majrad' is negative (and 'majrad' plus 'minrad'
  is positive) this defines a lemon.