Add optional Nu value in Stage0

As discussed with Michal, it would be nice if we can specify the Nu value in the stage0 file as an extra key.

Currently, the LbMCSubmit datatype is what automatically sets the Nu value of the production. However, with 2024 and UpgradeII coming, it would actually be quite useful if we have a Nu key that overrides the datatype, as it would be nice to produce those 2024 and UpgradeII samples with different values of Nu.

The Nu value is set by two things: a Gauss option file and the sim_condition.

The Gauss option files can be found here as an example*. We only need to choose which of the option files we need to emplace in the stage6 later on.

The sim_condition can also be algorithmically chosen. For example Beam3500GeV-2011-MagUp-Nu2-Pythia8 sets Nu = 2.

This means that implementing this change will focus on changing those two things.

Not urgent but would be nice if its done by December, preferably in 2-3 weeks or so.

cc @kreps

* They are specifically set in the option file with these lines (example from here)

#  It correspond to nu(total) = 1.51
Gauss().Luminosity        = 0.167*(10**30)/(SystemOfUnits.cm2*SystemOfUnits.s)
Gauss().TotalCrossSection = 101.5*SystemOfUnits.millibarn