changed material layers the other way around compared to issue 86

I changed the two models (outerWall and internalWall) the other way around compared to #86 . This way all structure models are consistent: layMul.port_b is connected to propsBus_a. I also added this to the documentation. Compared to the initial version only the internalWall model has changed so other structure records can remain unchanged. Since internalWalls are typically symmetric (?) I think the required changes are limited.

@saroele @damienpicard make sure to change your models when using this update?

Who else needs to be informed of this change? My apologies for any inconvenience caused..