Default value for nb_np_list parameter.

[thilinarmtb]

PR #15 set the default value of nb_np_list to 1 if the user did not provide any.
One problem I see with that is the given case may not run with a single MPI rank.
May be a better way to do this may be to set a nb_np_list value depending on
the machine. We can move the definition of the variable to appropriate machine
file and set it accordingly. For example, for linux machine type this can be 4,
for cetus, this can be 64.

@kent0 : What do you think?

[kent0]

It's true that the default value of 1 won't work if a specific machine has more than 1 core. In that case the user will have to specify the value/s anyway. It may be a good idea for it to be machine specific, but I think it's ok for the 'default' to be 1. We may want to use a model where default values get set in go.sh, but gets overridden by sourcing a machine specific variables file. We can also follow this model and add a user configuration file that will get sourced before the tests start.

[thilinarmtb]

Yes, this is exactly what I am talking about. Let's keep this issue till we add machine
specific defaults. Currently, the defaults are for a linux machine type.