Add optional LAMMPS dependency

[dschwen]

Let's figure out what the best way for coupling LAMMPS into Magpie is. Should we

a) add a LAMMPS submodule, like we're doing with SPPARKS? This will not work yet, as LAMMPS will not build with the strict compiler flags MOOSE is setting.
b) use LAMMPS as a library (we can use extract_fix to get pointers to internal lammps objects). This will make it harder to add custom LAMMPS objects (which we could have in the Magpie source tree otherwise). Basically users will have compile lammps with a custom add-on package (module).