ENPOGC ( curve, coords, tangnt, prnorm, binorm, curvat, ifail )
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  Enquire point on general curve.

Receives:
  KI_tag_curve             *curve          --- curve enquired of
  KI_vec_position           coords         --- position of interest on curve

Returns:
  KI_vec_normal             tangnt         --- tangent direction ratio
  KI_vec_direction          prnorm         --- normal direction ratio
  KI_vec_normal             binorm         --- binormal direction ratio
  KI_dbl_curvature         *curvat         --- curvature
  KI_cod_error             *ifail          --- failure code

Specific errors:
  KI_at_terminator          coords at terminator of curve
  KI_not_on_curve           coords not within resolution distance of curve
  KI_invalid_geometry       the curve fails to pass checks

Description:
  Calculates the tangent, principal normal, binormal and curvature of the
  curve at the given point. The first three are returned as direction
  ratios (normalised vectors).

  The application program should check the curvature, and ignore the
  normals if the curvature is very small.

  An example where this would occur is a straight line, which of course
  has a curvature of zero. In this case the principal normal and binormal
  directions are not returned.

  The curve 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).