Modifications to address IT199

[mdlpstsci]

• Improved messages
  • Fixed obsolete reference for the Drizzle software.
  • Improved the error message when the *_spt.fits file already exists for the association,
    and the "imclobber" environment variable is not set to "yes | YES". CALACS will stop
    with an error message providing the solution.

ERROR: Solution is to delete jcqtb4020_spt.fits file or to set environment variable 'imclobber' to 'yes'.

• Improved the warning message when looking for individual input *_spt.fits files.
  If the input missing *_spt.fits file is the file corresponding to the first *_raw.fits
  file in the list, this will cause calacs to error out (bad design). Now an error message
  is issued at this stage.

ERROR: Cannot find the first input file "jcqtb4a5q_spt.fits" which is needed to create the output file header.

If the input missing *_spt.fits file is any file other than the one corresponding to the 
first *_raw.fits, only a warning is issued.

Warning Cannot find input file "jcqtb4bnq_spt.fits", but processing can proceed. Output
association *_spt.fits is comprised of any found individual *_spt.fits files.

• Set pointers to NULL, used the return status from low-level functions versus setting
  (status = 1) explicitly, and freed the header structure after errors.

Resolves #199
Signed-off-by: Michele De La Pena [email protected]

[jamienoss]

@mdlpstsci Hey, does this still need reviewing?

[mdlpstsci]

I will have to re-check as it has been so long I do not recall.