Tweaking makefiles
Fixed issue with non-mpi example main not recompiling when it needs to
bugfix in non-mpi version; added support for short par & met names

When we added support for MPI runs, we broke running ABC without MPI.

Oops.

Made Metric a class instead of a struct, and gave it an interface
consistent with Parameter.

STDERR output now looks better & is a bit more useful, thanks to
short name support and reporting percent changes.