cvg: Remove debug write
cvg: New function to list a header to standard output
cvg: Do not use Fortran INQUIRE statement to see if a unit is open

Looks like FITSIO "unit numbers" are not real Fortran logical unit
numbers, and so the fortran INQUIRE statement cannot be used with them.
Use native FITSIO enquiry routines instead.
cvg: Fix bug writing out string columns

When writing a binary table, only the first row of a string column was
being stored successfully due to an error in passing the trailing string
length argument using %VAL.