Added control on exporting interior walls

User can decide if they want interior walls to be exported to radiance.
This makes it possible to use an energy model for daylighting analysis
with no need to change the model.
On the other hand not every interior wall is an air wall. Right now the
concept of air wall hasn't been really implemented to Honeybee so it
should be fine however it should be modified later to only remove
airwalls and not all interior walls.
I think I can close this issue now:
https://github.com/mostaphaRoudsari/Honeybee/issues/103
Modified normal direction check

In some cases Rhino generates closed brep with normal of surfaces facing
inward. Theoretically it should never happen but it does so I just give
a warning to user to take care of this issue.