plate folding to platonic solids

This adds a potential and system class for folding a set of plates into a solid.  For example, 4 triangles become a tetrahedron, or 6 squares become a cube.  The potential and system class are in the playground.  Related chages are

1. add a harmonic interaction potential
2. add a wca atomlist potential
3. change SimplePairwisePotential so that it uses add_energy_gradient rather then get_energy_gradient so it can be used in CombinedPotential.
4. NeighborList potential uses size_t instead of long int.