decrease integer literal indices on model input and output parameters

This replaces for example in the vertices

  ZP[gO1, 1] -> ZP[gO1, 0]
  ZP[gO1, 2] -> ZP[gO1, 1]

Still missing: self-energy expressions of the form

  -4*AbsSqr(CpconjVWmHpmAh(2,2))*B00(p,MAh(2),MHpm(2))

need to be replaced by

  -4*AbsSqr(CpconjVWmHpmAh(1,1))*B00(p,MAh(1),MHpm(1))