'reversed' connections in outerwall model

There is an inconsistency: in internalWall port_a of layMul is connected to propsBus_a. However in outerWall port_b is connected to propsBus_a. This is counterintuitive and inconsistent. Also I have made a component which is a combination of these two models and then I can not specify a building structure that works in both modes because the material layers are reversed. This pull request solves my issue but means that all construction layers in your models will need to be reversed. Therefor I understand if you rather not pull this, still I think it is the right thing to do! =)

If this is accepted I still need to change the existing constructionTypes in the library.