Add attribute for whether sub-unit is put in own category or parent category

[ibboard]

Currently, we put sub-units in the category of their parent. While this works for most situations, there are some situations where this isn't correct. For those units that can be taken as a sub-unit but don't automatically go in the category of their parent (e.g. a Heavy Support unit that can be taken by an Elite unit but which still counts its points towards Heavy Support), we need an attribute that says "unit goes in own category" (which should default to "false").