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      DOUBLE PRECISION FUNCTION slDPAV ( V1, V2 )

     - - - - -
      D P A V
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  Position angle of one celestial direction with respect to another.

  (double precision)

  Given:
     V1    d(3)    direction cosines of one point
     V2    d(3)    direction cosines of the other point

  (The coordinate frames correspond to RA,Dec, Long,Lat etc.)

  The result is the bearing (position angle), in radians, of point
  V2 with respect to point V1.  It is in the range +/- pi.  The
  sense is such that if V2 is a small distance east of V1, the
  bearing is about +pi/2.  Zero is returned if the two points
  are coincident.

  V1 and V2 need not be unit vectors.

  The routine slDBER performs an equivalent function except
  that the points are specified in the form of spherical
  coordinates.

  Patrick Wallace   Starlink   13 July 1997

  Copyright (C) 1997 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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