Peak refactor

Peak finding algorithm has been reorganized slightly so that calculations can be performed on a per-frame basis. See IData::FrameIterator for the details. Also included Biodiff instrument file, and fixed an integer overflow problem in background calculation (this was the reason that blob-finding was giving such strange results on biodiff data). For Biodiff data, the tool can now correctly find many (but not all) peaks, and from these calculate a reasonable unit cell.