MAINT: Update lammps input write in OPLS io

Status: Ready to merge

I want to write a test for io.opls.write_lammps() before calling this MR complete. This test will use an ASE Atoms object that has a cell that will be rotated in order to meet LAMMPS' cell orientation requirement so we can check that this is being done correctly. Edit: Done.

Summary:

93408c05 got rid of the function get_lammps_prism_str() and replaced positions_to_lammps_strs() with vector_to_lammps(). The OPLS io module was still using these old functions, so I've updated the calls here. Furthermore, I noticed that when writing the velocities out of the ASE Atoms object into the LAMMPS input file, they were not being rotated to match the LAMMPS cell's frame of reference (although the positions were being rotated correctly).

Closes #739 (closed)