RPREP: Read a Record from a Batch Surface Photometry Prep-File

RPREP reads a record from a batch surface photometry preparation file opened using OPREP. These files contain the following information (one record per image):

SKY value (into VISTA variable SKY)
max. radius for photometry (into variable RMAX)
estimated HWHM for the PSF (into variable PSF)
contents of the AXES block (into AXES)
image header (ignored by this routine).

This information must already have been written into the prep-file using the WPREP. OPREP opens the file, which must have already been opened read-only using the `OPREP R' command