only invoke pre_exchange() if neigh list stores more current history info

     than npartner/partner arrays in this fix

   that will only be case if pair->compute() has been invoked since

     upate of npartner/npartner

     update of npartner/npartner

   this logic avoids 2 problems:

     run 100; write_restart; run 100

       setup_pre_exchange is called twice (by write_restart and 2nd run setup)