fixed smoothing bug and improved height computation

Layer height smoothing on feature edges did not work due to odd-even
decoupling. The current system is based on boundary codes and needs to be
change din order to work with the new surface algorithm (starting from
enGrid 1.5).

The ratio between bottom and top area of a prism can now be limited.
The angle between these two faces can also be limited.
(m_FaceSizeLowerLimit, m_FaceSizeUpperLimit, m_FaceAngleLimit)