lammpslib calculator and lammps data file reader changes

Bug fix in LAMMPS data file reader: If create_atoms is set to False, then self.previous_atoms_numbers is never created and an error is thrown on the "do_rebuild" calculation on line 368.

Feature add in LAMMPS data file reader: Gave user ability to specify units in LAMMPS data file prior to read

Test added for lammpslib calculator: Added test for lammpslib in which a datafile is read in