Make q, u default Matrix properties

[tjstienstra]

While I did choose to use q and u all the time, I noticed that if there is only a single one of them, that I just directly made it a dynamicsymbol. This is however a bit inconvenient, because this means that the type can be different of the same method name in different models and connections. Therefore, I propose adding a q and u property to BrimBase, similar to symbols.