_stepCount internal env prop should not be mapped

[Robadob]

It appears that the step counter's internal environment property is being mapped to submodels.

This causes any agent functions (and host functions) after a submodel, before the end of the step to report the incorrect step value when FLAMEGPU->getStepCounter().

Confirmed the problem, by updating the step counter env property after the submodel returns. However, the proper fix would be to block it from being mapped in the first place.

I would presume agent _id var wants to always be mapped. But there doesn't appear to be anything explicit in the code to handle this so far, just all existing usages use automap which maps all matching.