fixes

from #136 : the joint segmentator had a bug for which ids were not correctly assigned to each faces.
assembly segmentation: minor but mighty changes in the C++API to improve it:
DFMesh::IsPointOnFace():
- The coordinates u, v on the triangle face are not really barycentric coordinates. The comment in the code has been adapted.
- Given the nature of u, v, a small deviation can lead to large difference in 3D coordinates. For that reason, the association threshold is divided by 100 for more precise manipulation
DFSegmentation::AssociateClustersToMesh() and CleanUnassociatedClusters():
- The angle between the face normal and the line that joins the face center and the cluster center is taken into account. Ideally this angle must be very close or equal to 90°. A condition states that the cosine of this angle must be smaller than the angle threshold.

new features