Fix nblib pairlist update function

Previously the function put_atoms_in_box was called only by the
nblib integrator, or when constructing a system via a SimulationState
object. In the case of the integrator, this is a needless performance
degradation. When using an nb force calculator without
first putting atoms in the box, this could lead to a cryptic
error from nbnxm grid search failure. Both of these issues are
rectified with this change, which also adds a member to the
non-bonded force calculator to hold the requested number of OMP
threads to use in a call to put_atoms_in_box_omp, which is faster
than the non OMP version.