Bond breaking symmetry check

[pabloseleson]

We need to check whether bond-breaking is done so that symmetry is no broken. I.e., forces are not computed for some particles before the relevant bonds are broken.

For example, in PDMATLAB2D we have for time integrator:

% Step 1: Compute velocity at t+dt/2 for all nodes

% Step 2: Compute displacement at t+dt for all nodes

% Update neighbor list if bond breaking occurs

% Compute internal force density and macroelastic energy density at t+dt for all nodes

% Step 3: Compute velocity at t+dt for all nodes