Draft: Resolve "Adding stablizing terms to the force constant model for MD/MC purposes"

A number of stabilizing calculators have been implemented:

• OnSiteCalculator: defines an artificial potential binding the atoms to their lattice sites. Think of it as a general Einstein crystal potential. Has no knowledge of orbits or species.
• SimplePairCalculator: defines a pair interaction among all atoms and treats all atoms as the same species and with no knowledge of orbits.
• PairPotential: Orbit based interaction based on linear combinations of a user defined basis functions. Only works for pairs at the moment but can be modified to define an onsite interaction too. An auxiliary function calculates a sensing matrix given an atoms object with an attached PairPotential.
• ZBLCalculator: The proposed zbl interaction. The prefactor is most likely wrong
• InvCalculator: old implementation which can be implemented as a SimplePairCalculator now.

The PairPotential is probably the most interesting and an example is provided in the examples directory under stabilizing_md/pair_pot.py

Your initial thoughts?

Closes #256 (closed)