Relaxation dynamics: capture singularity points from the geometry

[spadaxsys-dev]

The code should be able to capture the singularity points in the geometry.
The idea is first check all key points in the geometry by checking its curvature which is computed from the level set function. Then, for each key point with large curvature, we create a new singular particle. We may need to check whether this new particles is too close to an already created singular particle according to a threshold to avoid too fine geometry beyond the resolvability of particles. These newly created particles will be tagged as constrained particles so that it does not move in the relaxation process.