Draws heliocentric grid over an image. It reads the ATT file to get
information on the spacecraft pointing, and also calls GET_RB0P to get
the solar radius. A sample call could be:
IDL> tvscl, data(*,*,0)
IDL> sxt_grid, index(0)
The keyword option /read will allow the user to move the
cursor around the image and to display the heliocentric coordinates.
If /angle is used with /read, then the value displayed
is the angle relative to the sun center, which is useful to be used
with SXT_PREP.