implement gam2 process

See merge request !38

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# Conflicts:
#   src/partons/modules/evolution/gpd/GPDEvolutionApfel.cpp

Fixes in GPD evolution by APFEL++ and incresing the number of nodes in the x-space grid of Vinnikov's code

Moved initialisation of alpha_s and x-space of APFEL for GPD evolution from prepareSubModules into initModule

ek[1] -> ek[0]
ek[2] -> ek[1]
ek[3] -> ek[2]

photon (on m_phi - angle between polarization and transverse momentum of
the outgoing photon).
Modifications are commmented and tagged as "TODO"

Commented messages in GAM2CFFStandard