Diffusion Processing Pipeline/bugs and feature requests

• mrifilt3, mricrop, mrisubset, and possibly others mangle the parameters file. In particular, if you operate on a DWI, the output won't have the b matrix recorded. Also the MRISpaceShift just gets clobbered to 0 0 0. A workaround right now is to just copy the old image's parameters file, but the latter two programs change the scale and offset.
• Streamtube generation gets slower and slower as the program runs. What's going on? Is it running out of memory, reading inefficiently through a big file, doing something probabilistic that gets less likely the more tubes there are? We need a speedup!