Option to not include wall forces when pickling the forcefield

[chrisjonesBSU]

Right now, when pickling the force field, the entire list of forces is saved. Sometimes a force is added automatically behind the scenes (adding walls for slab and weld simulations) that we don't want to reuse in a separate simulation. Right now, wall forces are the only ones automatically added, so I'm thinking we just have a parameter like exclude_walls in the pickle_forcefield() method.