Use less neutrons and remove sections where we increase number of neutrons

[nvaytet]

• We replace the old versions of the mcstas notebooks with the short ones
• Use less neutrons in the mcstas notebooks because we don't need so many statistics as we are not doing analysis

[nvaytet]

It all seems to run fine (in half the time!).
Only thing that appears to be broken is the QENS analysis, where the fit fails:

But since we are not doing the analysis part, I'm fine with leaving it as is, and trying to fix it later.