Diffusion Processing Pipeline/bugs and feature requests

• mrifilt3 and mricrop mangle the parameters file. In particular, if you operate on a DWI, the output won't have the b matrix recorded. A workaround right now is to just copy the old image's parameters file.
• 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!