Draft: LbESEPP for beam-electron scattering

Request for integration in Gauss of a new generator, ESEPP, to simulate the elastic lepton-proton(/nucleus) scattering taking into account the radiative corrections. This is a rebase, with modernised code, of MR !458 (closed) from 2019 (which was closed)

See details of generator in presentation on 12 October 2022