[WP][DOC] (issue7_refract): add introductory comment

Explain the different rectangle definitions used with in settracevp.

src/ts/refract/sub/refract_settracevp.f

@@ -57,6 +57,8 @@ c
       include 'refract_para.inc'
 c
 cE
+c
+c ----------------------------------------------------------------------
 c 
 c tov stands for total viewport
 c trv stands for trace viewport
@@ -70,7 +72,65 @@ c
 c the result flag is used to check whether any part of the curve is
 c visible within the global viewport
 c 
-c the baseline of the curve must be 0. or average
+c the baseline of the curve must be 0. or averagec
+c
+c ----------------------------------------------------------------------
+c
+c The function calculates the viewport coordinates for the rectangle on
+c the graphics device display to which the given trace signal shall be
+c plotted, the trace viewport. These viewport coordinates are:
+c
+c   left:   trv_vpleft        in normalized device coordinates
+c   right:  trv_vpright       in normalized device coordinates
+c   top:    trv_vptop         in normalized device coordinates
+c   bottom: trv_vpbot         in normalized device coordinates
+c
+c The total viewport for the complete graphical display of traces is
+c defined by the rectangle
+c
+c   left:   tov_vpleft        in normalized device coordinates
+c   right:  tov_vpright       in normalized device coordinates
+c   top:    tov_vptop         in normalized device coordinates
+c   bottom: tov_vpbot         in normalized device coordinates
+c
+c The subroutine make sure that the trace viewport is inside the total
+c viewport. 
+c
+c ----------------------------------------------------------------------
+c
+c The world coordinates for the total viewport are:
+c
+c   largest offset:     tov_rmax    in meters
+c   smallest offset:    tov_rmin    in meters
+c   largest time:       tov_tmax    in seconds
+c   smallest time:      tov_tmin    in seconds
+c
+c With respect to these world coordinates in total viewport the world
+c coordinates of the trace viewport are
+c
+c   largest offset:     trv_rtop    in meters
+c   smallest offset:    trv_rbot    in meters
+c   largest time:       trv_tright  in seconds
+c   smallest time:      trv_tleft   in seconds
+c
+c ----------------------------------------------------------------------
+c
+c The actual world coordinates to be set to the window covering the
+c trace viewport are:
+c
+c   left:         trv_tmin    in counts
+c   right:        trv_tmax    in counts
+c   top:          trv_vmax    in seconds
+c   bottom:       trv_vmin    in seconds
+c
+c ----------------------------------------------------------------------
+c
+c The subroutine first sets the vertical extend and window in the offset
+c domain and world coordinates in counts. In the second part it operates
+c on horizontal coordinates and world coordinates in seconds.
+c
+c ----------------------------------------------------------------------
+c
 c 
       logical result
       real vppm, ttmin, ttmax, theoffset