check that nStartIntr >= 1

[PeterJacko]

Function lNewIntr checks whether vArrTimes has at least 1 ISA to be added at time 1 to make sure that at least one ISA starts at the beginning of the trial, but this is only checked when !is.null(vArrTimes); otherwise there is no check, so I think that we should add checking that nStartIntr >= 1. This is important for making sure that length(lPltfTrial$isa) >= 1 from the beginning, which is often assumed in the rest of the code.