Redefine clean, clobber and new targets in Makefile.common

[mandli]

I am a little confused as to the purposes of each of these targets, in particular clean. Is clean meant to clean out the local directory data, output and plots (it does not do this right now)? Is clobber supposed to both clean and remove all compiled objects (it does do this and more)? Should new really just clobber and make the executable again?

I would propose that we redefine these options as I suggested above to make it clear what exactly the roles of these phony targets are.