Adding potential_utils module and additional examples for the PotentialBlocks

I push some utility functions I wrote during my PhD to simplify the usage of the PotentialBlock shape class. These allow to create easily:

• Generic PotentialBlock particles (I wrapped the body definition, as we do for the polyhedra class)
• Cuboids
• Platonic solids (tetrahedron, hexahedron, octahedron, dodecahedron, icosahedron) defined by either of their volume/edge length/inradius/midradius/circumradius
• Prismoids of variable radii on their two ends and variable number of faces
• Cylinders, made as prismoids of many faces
• aabbPlates surrounding a cuboidal volume and cylindricalPlates, to generate equivalents to aabbWalls and facetCylinder, but using PotentialBlock particles.

I have added examples to simulate cuboidal and cylindrical packings within bounding plates, a multi-drum column, and some standard shapes generated using the new functions.

At the moment I am having a Segfault when trying to simulate icosahedra in contact, and I have not managed to trace its origin. Any help is very welcome!!