RAYFIR ( palist, transf, wchhit, point, direct, nhitpt, hitpts, hitfas, 
========================================================================
         indces, ifail )
         ===============

  Intersect ray with bodies

Receives:
  KI_tag_list_part         *palist         --- entities to fire ray through
 <KI_tag_list_<transform>> *transf         --- part transformations
 <KI_int_nitems>           *wchhit         --- number of intersections wanted
  KI_vec                    point          --- point from which ray is fired
  KI_vec_direction          direct         --- direction of ray

Returns:
 <KI_int_nitems>           *nhitpt         --- number of points returned
  KI_tag_list_dbl          *hitpts         --- points where ray hit bodies
  KI_tag_list_face         *hitfas         --- faces hit by ray
  KI_tag_list_int          *indces         --- indices of bodies hit by ray
  KI_cod_error             *ifail          --- failure indicator

Specific errors:
  KI_wrong_transf           Unsuitable transform
  KI_not_same_length        Entity and transform lists are of different lengths
  KI_wrong_entity_in_list   Entity is not a part

Description:
  RAYFIR fires a ray at a list of transformed parts and returns the faces hit.
  A face is considered to be hit by the ray if and only if the ray passes
  through the face. Cases where the ray touches the face but does not pass
  through it are not considered hits, e.g. a tangential intersection of the ray
  with a face is not a hit.

  This is achieved internally by first applying the inverse transform to
  the ray, and then applying the original transform to the hit points found.
  If none of the parts are to be transformed, 'transf' may be specified as a
  null tag, which will be interpreted as an identity transform.

  The ray begins at 'point' and proceeds along 'direction' as far as necessary:
  faces in the opposite direction will not be found.

  For each intersection found, the routine will insert in the lists it returns:

      The coordinates of the point of intersection, in the next three entries
      in 'hitpts'.

      The tag of the face intersected by the array, in the next entry
      of 'hitfas'.

      The index in 'palist' of the particular part owning the face hit by the
      ray, in the next entry of 'indces'.

  Thus the lengths of 'hitpts', 'hitfas' and 'indces' will be 3*'nhitpt',
  'nhitpt' and 'nhitpt' respectively.

  If the ray does not intersect any of the transformed parts three empty lists
  will be returned.

  If non-empty lists are returned then they will be sorted according to ascending
  distance measured from 'point' along the sense of 'direct'. If the ray hits
  two faces at the same point, for instance at an edge, then the relative
  ordering of the pair is arbitary.

  The number of intersections returned is controlled by the parameter 'wchhit'.
  At present the values are:

      0      - All intersections with all bodies
      n > 0  - The closest n intersections with the ray to the point
               from which the ray is fired along the sense of 'direct'.

  If transforms are supplied they may only contain translation and rotation
  components. Reflections, scales and shears are not allowed.