Allowing option of single-core "multicore" running in MGC
Athena understands and respects the difference between an unset
ATHENA_PROC_NUMBER
(serial running) and ATHENA_PROC_NUMBER=1
("multicore" running with a single core) as a way to distinguish modes
of running but still run a single core. This MR adds the same feature to
MadGraphControl, so that it uses the multicore running model if
ATHENA_PROC_NUMBER=1
is set and the serial running model if it is unset.
Thanks @mhance for the suggestion!