raw_general_transform, and all interfaces to it, now takes nthr as an optional...

raw_general_transform, and all interfaces to it, now takes nthr as an optional parameter. It has been extensively tested. Note that the code has also been extensively restructured, which means that even existing code will run a different version of the functions in warpfns. For that reason I have attempted to test every path through the code. But we should be aware that the underlying code has changed for all resampling of images.