Put back LbPowheg into 'master'
This reverts merge request !542 (merged)
"suspending LbPowheg" due to library for gcc99 now being available in LCG 96b (see info SPI-1506
This is related also to MR !536 (closed)
Integrated in !577 (merged)
Closes LHCBGAUSS-1929
Edited by Gloria Corti