Several fixes and improvements for multi-dim fits

 - Implement special combine options for --stable
 - Add instructions in sm-protocol for patching combine and running fits
 - Modify src/MultidimFit.cc to catch some cases where extra added points
   would have been in the wrong place
 - Updates to the plotting layout files
Merge pull request #251 from ajgilbert/2d-scans

Several fixes and improvements for multi-dim fits