New scheduler which activates a subset of all agents by condition

[Tortar]

We still don't have a scheduler capable of such a thing, but this could be useful in some situation where for example we want to activate just a group of agents with certain properties, for example in a ForestFire model this could be useful since only the burning trees are actually activated. But there are different ways to implement this new scheduler, from the most obvious implementation where we filter all the agents each time to some more complex ones which need to be carefully evaluated and can be more difficult to deal with for the user but could be more efficient, see some last comments in JuliaDynamics/ABMFrameworksComparison#13