Fix bug in calculation of acceptance rate.
Fix bug in calculation of acceptance rate.

In cases where the previous count of accepted moves is higher than the
current (can happen in new runs) we returned a negative value. Now we
disregard the previous counts and return the correct acceptance rate.

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue