MCFM 6.8 and various comprehensive updates

Updates log:

1) TVar naming conventions more general, self-defines are possible now (also needed for vvcoupl vs wwcoupl in vbfMELA)
2) Major changes in how TEvtProb works, see testME calls for how it is used now
3) Running renormalization scale from MCFM 6.7 was missing, added it
4) wwcoupl can be decoupled from vvcoupl if one defines any non-zero array elements
5) Makefile update
6) Readme update, also included libmcfm_6p8.so directly (you may remove it later if you want)
7) Added all files needed to be replaced in default MCFM files under ZZ_MCFM/src/*. Instructions also on Readme
8) Additional .dat/DAT files if/when needed

U. Sarica