Return the name of an image filter.

% image_filter --help 

image_filter: 
 Query a FITS image header to find the filter name. If no suitable 
card is found, and the query flag (arg2) is "Y", then the user is 
interactively queried for a filter name. 

Currently recognized header cards (in order of significance): 
  FILTER 
  INSFILTE 

The name of the filer is written bot to standard out AND a local 
file named image_filter.out. All other temporary files should be 
cleaned away after a full run of the script. 

EXAMPLES: 

% image_filter
Usage: image_filter ./S/20180206T022042.6_acm_sci_proc.fits Y 
arg1 - Name of FITS image (can be full path)
arg2 - run in interactive mode if necessary (Y/N)

% image_filter /home/sco/sco/Red_PhaseI_Data/jun_2005_p1/Rsco5261.fits N
R

% image_filter /home/sco/A_wcsf/Run_apr17/local_red/WCS/20180206T070902.2_acm_sci_proc.fits Y
r`