Add plotting

Added options making preview images:

mpiexec -n 1 vfisv --path='/dat/sdc/gris/20150919/level1_split/' --id=3 --line=15648.514 --numproc=20 --width=1.8 --out='gris_20150919_003_inversion.fits' --preview='gris_20150919_003_preview.png'  --errors='gris_20150919_003_errors.fits'

if there are more than 1 map, the output preview file will be a animated gif file gris_20150919_003_preview.gif