Add initial-state p_z and inelasticity selectors for DIS
@andriish requested the addition of the following two selectors to the framework:
IPZIN fl min max
INEL fl1 fl2 min max
They operate partly on initial-state particles, such that the universal selector can not be used. The first cuts on the absolute z momentum of an initial-state flavour fl
and the latter on y_12 between flavours fl1 and fl2 (either both initial-state or mixed), with
y_ij = 1 - p_j,0 / p_i,0 * (1 + costheta(p_i, p_j)) / 2
He has also provided code snippets for different Sherpa versions:
- 2.2.x: Standard_Selector_DIS_220.C
- master/2.3.x: Standard_Selector_DIS_230.C