This fixes some problems with halfspace (=planes) in mesapd:
constructor provided a default normal which does not make sensethe current implementation of sending shapes requires pack/unpack to be called on an actual object of the shape type, this requiring a default constructor.
- the provided normal is not further checked if it is normalized, and thus now calls normalize to ensure this
- the contains and ray-intersection still accounted for a possible half space rotation which according to the doc of the half space should not be the case. This is thus removed.
- documentation fixed and improved in some points