Draft: Add the new Jpsi -> 4 leptons model

This MR adds a new EvtGen decay model that can be used to model Jpsi decays to 4 muons or 4 electrons (leptons of the same flavour).

The model is borrowed from the BES III collaboration in agreement between the physics coordinators of the two collaborations. In BES III, this model was used as a combination of the "EvtDIY" model (Do It Yourself, that allows to plug any computed amplitude) and the (very long) amplitude expression that was coming in a separate .C file. The latter was adapted by Jianping Dai (BES III) to the LHCb case of unpolarised Jpsi production. I have refurbished the code and performed numerous checks with it. It was checked that the Jpsi->4e and Jpsi->4mu simulations produce reasonable distributions. I also added checks for the spins of the particles etc.

Usage:

Decay J/psisig
  1.000         e+  e-  e+  e-      PsiLLLL 3.097 0.000511;
Enddecay

or

Decay J/psisig
  1.000         mu+  mu-  mu+  mu-      PsiLLLL 3.097 0.105658;
Enddecay

In principle, the model should also allow to simulate psi(2S) decays – thus the generic name and the need to provide the vector-meson mass as an argument; this was tested on 5 events but no check of the distributions was performed so far.

cc @lshchuts

Marked as a draft before some final checks are completed.