Minor fix in clean target.

• Introduced a new validation procedure: bin/rivet-mkvaldir will

now create a Makefile in the given directory based on the
"Validation:" section of all .info files. "make check" will
download all necessary HepMC files (if they have not already been
downloaded), produce the validation yodas and compare them to
reference ones in the {prefix}/share/Rivet/refyodas directory. If
differences are found the corresponding plots will be produced
using rivet-mkhtml. For more information do "make help" in the
created directory.