Start for an interface between neutrino generators: GENIE, NEUT, GiBUU, NuWro, ...

This essentially is based on neutgeom from NEUT, with some first work to include GiBUU. Including GENIE should be fairly straightforward while for NuWro a separate function that calculates xsections is needed.

Ideally: pre-calculate xsec splines with different model variations for each generator and interface the spline interpolation with TXSec.

Currently: event_rate.cc: Calculated total cross section and probabilities for each neutrino flux vector in each part of the input geometry. INPUTS: a TNeutrinoFlux a TDetectorGeometry a TXSec: generator specific. Given neutrino type, energy, A and Z, channel. Interface for NEUT ready, first version for GiBUU in beta testing. => Should all be spline lookups for speedup. -> used by event_rate.cc through TNuTrajectory

		  OUTPUT : Rootfile with precalculated probabilities.

  generate event (genev.cc): Use precalculate probabilities to choose interaction vertex and generate 
  	   	 	     kinematics and final state particles.
			     INPUTS: TNeutrinoFlux
			     	     TDetectorGeometry
				     Rootfile from event_rate.cc
				     The type of output, eg. RooTracker, something flatter, a Les Houches, ... TMCOutput
			     uses TNuGenerator which interfaces with all generators to generate kinematics and
			     prepare output initial/final state particles for output format.

To Compile: same as standard neutgeom.