submit_template is now a per-test variable.

It is useful to have different submit templates for calculations of
different sizes (e.g requiring different numbers of nodes).  A default
can still be set in userconfig.

Also respect min_nprocs and max_nprocs defaults (as promised in the
documentation...!).
Force new benchmarks to always begin with benchmark stem.

This works around the case where benchmarks are create with the test
stem by accident if the -bt:XXX option is used.
Correct name in documentation for the diff action.

Reported by José María Escartín Esteban (TCM, Cambridge).