Implementation of Volume-interacting Level-Set DEM or VLS-DEM (2/3)

Hereby the Ig2 file for the implementation of VLS-DEM.

Particle-particle and particle-wall contact are included. Particle-sphere and likely particle-cylinder will be added later.

The code is rather heavily commented, but I thought it would be helpful given the complexity of the code, especially solving the intersection of the bounding ellipsoids is tricky. See publication mentioned in !961 (merged) for a more comprehensive explanation.