Added the EvtBLLNuL (BLLNUL) model; removed EvtB2MuMuMuNu.

The EvtBLLNuL model generates rare B -> ell ell nu ell decays,
where ell = e or mu; courtesy of Anna Danilina and Nikolai Nikitin (LHCb).

Removed the EvtB2MuMuMuNu (BUTOMMMN) model, since its now replaced by the
more general BLLNuL one.