ENPOGS ( surfac, coords, normal, prdir1, prdir2, prcur1, prcur2, ifail )
========================================================================

  Enquire point on general surface.

Receives:
  KI_tag_surface           *surfac         --- surface being enquired of
  KI_vec_position           coords         --- position of interest on surface

Returns:
  KI_vec_normal             normal         --- surface normal
  KI_vec_direction          prdir1         --- principal direction1
  KI_vec_direction          prdir2         --- principal direction2
  KI_dbl_curvature         *prcur1         --- principal curvature1
  KI_dbl_curvature         *prcur2         --- principal curvature2
  KI_cod_error             *ifail          --- failure indicator

Specific errors:
  KI_at_singularity         coords at singularity of surface
  KI_not_on_surface         coords not within resolution distance of surface
  KI_invalid_geometry       the surface fails to pass checks

Description:
  Calculates the normal and principal directions, and the principal
  curvatures, of the surface at the given point. The normal and principal
  directions are returned as direction ratios (normalised vectors).

  If the principal curvatures are equal, the principal directions are not
  significant.

  The surface must be capable of passing the checks imposed by CHCKEN.
  The self intersection check is only performed if the appropriate option is set
  (see SEINTP).