Fix handling of LCG_generators file
!163 (merged) caused a regression in lhcb-lcg-devXlhcb slots.
The line
list(REMOVE_AT _generators_data 0 1 2)
was flattening the list _generators_data
which should be a list of lists.
This change avoids the problem altogether by making the handling of special lines in the LCG files more robust.