wsclean.py: Remove -image.fits when only create dirty image
wsclean.py: Remove -image.fits when only create dirty image

And some minor tweaks.

liweitianux/atoolbox,liweitianux/atoolbox,liweitianux/atoolbox,liweitianux/atoolbox,liweitianux/atoolbox,liweitianux/atoolbox