ValidateIDCollisions runs when not required

[ptheywood]

The ValidateIDCollisions method / kernels(s) run when agent populations are set.

If the population was created at runtime (i.e. not loaded from disk) and theh population was empty originally then there is no opportunity for a collision afaik, so this can be skipped.

For a population of ~ 30k running on a 1070 (100% utilisation) this will save ~150us per population, which is generally insignificant but still an improvement in work efficiency.

So low priority, but probably worth doing eventually.