Simulate trigger channels first

[christophwelling]

In RNO we are going to have around 20 channels, but we are usually only simulating triggers on the four channels of the phased array. Since most simulated events won't trigger, it would be nice to be able to first simulate the signal for the trigger channels, check if there is a trigger and only simulate the other channels if there actually was a trigger. This would speed up the simulations by quite a lot.