Added par_mpi.c and inc/onetimesimu.h, which enable the parallel usage
of more than one computing node by using mpi.
Corrected an error occurred when
1. PXend isn't a shared variable
2. left!=0 but nCal=0, other threads may affect the assignment of PXend
Merge branch 'master' of https://github.com/francktcheng/BSERROR