Added --featout option and fixed buglet

I have added a --featout option that will generate two files. If one specifies --featout=my_fname the files will be named my_fname_fieldmap_rads and my_fname_fieldmap_mag, where the first is the field in rad/s and the second is the average unwarped image. These can be directly used in FEAT. In the process of doing that I discovered a "buglet", which meant that, even in the absence of movement, the most basal slice would get replicated at the very top. This had been reported as a bug, but I had not seen it because of the masking that is done. But it did mean that there would always be a missing slice at the top, which could have been a problem if the FOV was very tight in the slice direction. And it prevented me from outputting an unwarped image that wasn't truncated at the top and/or bottom. I have now fixed the bug, which means that my_fname_fieldmap_mag will have full FOV. I also added writing of a mean unwarped image as part of the --iout output, also with full FOV. This will be useful for creating a mask for eddy and/or dtifit. It means that for data with very tight FOV the estimated field will be slightly different, especially in the topmost slice. But the field generated by the new version will be, slightly, more accurate.