Validate for concavity

[iulian787]

When a mesh is validated (before intersection / overlap for example), the concavity test is against 180.
That would translate to that even a slightly concave mesh (an internal angle slightly larger than 180 degrees) is "invalid"

We also use validation for meshes resulting from intersections, and they get rejected for a slight concavity, let's say, around 180.00000000001 degrees

ReferenceTolerance is for distances, which is set to around 1.e-12 on unit sphere. Basically, if we express in radians, an angle less than 1.-e12 radians should not invalidate a mesh; or pi+ReferenceTolerance should not be considered concave.

test at line 1730 in GridElements.cpp is for a dDot against 0.0; it should be someting equivalent to a reference tolerance of 1.e-12 radians;