[IMP-dev] No unused stats

I removed the unused stats code in ModelData.h. We can do the same things through ScoreStates (see MaxChangeScoreState)

It also removes the flag which is set when particles are activated or deactived since that is not used any more. If we have one, it should be in Model anyway.

And moves the vector include to after the IMP includes so it doesn't mask any missing vector includes in them.