Revert "parallelize simple_mpcd_step"
Revert "parallelize simple_mpcd_step"

This reverts commit 671b7fed8a2d01987a5d28830943ca9228ae8e85.

pdebuyl-lab/RMPCDMD,pdebuyl/RMPCDMD,pdebuyl-lab/RMPCDMD,pdebuyl/RMPCDMD,laurensdeprez/RMPCDMD,laurensdeprez/RMPCDMD