inpainting

[ntustison]

@stnava, @cookpa The most recent commit includes inpainting for t1 and flair whole head images. I think this is good for a first pass but there's probably room for improvement. I'll work on porting to ANTsRNet.

@stnava, this development led to some bells-and-whistles I'd like to add to our core u-net implementation. We should discuss sometime.