Minor changes for ihixs purposes

ihixs has effective implementation of delta and plus terms.

Concerning the core/tool files:
vegas_adaptor.h: moved vegas variables from private to protected.

luminosity.h: added double -almost_zero
luminosity.cpp: implemented a security test: if x1<0 or >1 or x2<0 or
>1 the give(x1,x2) returns zero. This helps safeguard against bjorken
xs generated irregularly, but also allows to implement a more brute
generation of z from 0 to 1 instead of from tau to 1.

model.cpp: added a string to indicate in runtime up to which order is
the running of couplings and masses performed