new LCG layer LCG_104c_ATLAS_3

A change to new LCG layer requested and discussed in https://sft.its.cern.ch/jira/browse/SPI-2465 Contains: Pythia308, installation of MCFM and also a re-installation of Sherpa 2.2.14p1 (+ MPI-enabled Sherpa) with the additional —enable-mcfm flag in both cases . GRPC: done Madgraph5amc 3.5.2 with patches reviewed and updated by zmarshal apfel 3.1.0 with new CMake build system. yoda-1.9.9 + rivet-3.1.9 @elmsheus @nstyles @jchapman @jcatmore