[IMP-dev] RMF backward compatibility

I removed some code from the RMF library that provided backward compatibility with certain rather old files (1 year+ I think). If you run into any files that don't open after you update IMP and they matter, I can put the code back (I rearranged some functions and would have had to update the code). Alternatively, if you know you have some old files, you can do rmf_slice old.rmf new.rmf -f -1 which will update the files into new.rmf (the -f -1 just means move every frame).