Add "RequiresNoMoreThanNUnitsInParent" requirements

[ibboard]

Sometimes, a unit has a range of sub-units but is only allowed one at once (e.g. multiple monstrous mounts). Currently, we don't have a way to control this - requirements like RequiresNoMoreThanNUnits are global. What we need is a similar thing, but just checking the children of the unit being added to so that we can have mutual exclusion of mounts.

The alternative is to have something at the ContainsUnits level that groups them and puts the requirements on from the top down.