Added functionality to support Ring polymer initialization for PIMD

[xmwebb]

• member function 'preparePIMD' was added to State to map existing atoms, molecules, bonds, and multi-atom potentials in state to RP representation.
• member function added to Atom to generate Cartesian coordinate positions from ring polymer normal modes. This is used for initialization of positions consistent with centroid at classical position assumed in State
• virtual member function added to Fix called updateForPIMD, which takes existing forcers and replicates them for new atoms needed in ring polymers. Functions that do things of this kind added to FixBond and FixPotentialMultiAtom