Add unit size constraints that only apply when a sub-unit

[ibboard]

Some units (e.g. Space Marine Attack Bikes) can be a small squad on their own or they can be contained in another unit. In these cases, the unit size might change (Attack Bikes can be a unit of 1-3, but only one can be taken in a normal Space Marine Bike Squad). This should be handled on a per-parent type basis, to allow people to re-use units in this way.