ROOT/GSL integrator

[forthommel]

This PR introduces a new "event_generator" module implementation, based on ROOT's Math::IntegratorMultiDim.

It allows to compute cross sections with more numerical stability than the usual Foam method, thanks in particular to ROOT's C++ wrappers to GSL's Vegas, Miser, and plain Monte Carlo integrator algorithms implementations.

This results in shorter runs for most algorithms used, and comparable or better precisions reached for a same run time. For instance, running the epic_scenario_DVCS.xml example on an 8th Gen Intel CORE i7 vPro with 16 Gb RAM:

• Foam (with 100k events generation upstream, 22'45" run):

  Integrated cross-section (value) [nb]: 4.745535713297
  Integrated cross-section (uncertainty) [nb]: 0.0190818197995742
  

• Plain (no event generation, 1'17" run):

  Integrated cross-section (value) [nb]: 4.88435227940284
  Integrated cross-section (uncertainty) [nb]: 0.324427311999416
  

• Miser (no event generation, 1'38" run):

  Integrated cross-section (value) [nb]: 4.71182617972158
  Integrated cross-section (uncertainty) [nb]: 0.0555660423376965
  

• Vegas (no event generation, 8'59" run):

  Integrated cross-section (value) [nb]: 4.74743323842268
  Integrated cross-section (uncertainty) [nb]: 0.00602654005686526
  

[JJJhonzon]

de

[forthommel]

Hello @pawelsznajder.
In this last commit a35715e, we try to minimalise the changes in the EPiC workflow by computing the cross section at the initialisation stage, instead of the end of the run.
The PR is now ready for your review!

FYI: @hamzeh-khanpour

[forthommel]

Many thanks for your fast reaction, @pawelsznajder!

[pawelsznajder]

I would not call it very fast ;) I totally forgot to make the merge after our last discussion, sorry for that. Anyway, the new release (including your changes) should be ready in few minutes.