initial implementation of the strain induced potential. The setup is
working for integer sites. Todo: saddles. Fixed a bug where you could
not set the position of a particle unless you had previously tried one
in debug mode.
implemented test for reverste engineering the strain and checking that
the displacement is correct. passed.
saddle point calculation test implemented and passed.
added map for tracking which particles are affected by the strain. Such
that if some neighbor decides to clog others, by adding / removing their
surface property. Easy to check if something has been changed.