GitLab

 - methods primitiveGradient and potential are now defined in primitives (CompactPlane, AlgebraicSphere, MongePatch)
 - primitives can now expose cast function to give explicit access to methods potentially hidden by the CRTP inheritence

Add curvature estimators, fix several warning and improve testing.

See merge request !1

add optional parameter to compute the tangent plane either from dN, or from N

- test of dNormal() with plane and sphere

- test of consistency of dPotential() and dNormal() wrt to scale/space derivation