implemented memory in the stress interaction. Only if the particle is a
surface particle it is affected. If a neighbor interferes with the
particle state, additional energy is not added if it has already been
taken into account.
a surface moving can change its stress interaction without changing its
particle state. It is now taken into account.
preparations for implementation of stress potential into class. TODO:
rename strain stuff more apppropriately. Rename potential to
interaction. Incorporate stress interaction into solver. Finish test.
Move electrostatic interaction into new interaction class setup. Update
tests. Merge. Implement app for Dag's case.
added license and readme