fix overlapping send and recv buffers

[mawi2017]

Fixes #18 overlapping send and recv buffers. Now different buffers for MPI send and Irecv are used that happen at the same time.

I compiled and tested only a limited subset of the compilation targets. The ones I tested look good so far:

• ref/src

  • gcc 11.2.0

  • Intel(R) MPI Library for Linux* OS, Version 2021.7

  • ITAC 2021.6.0

  • command: make clean && make -j && mpiexec -n 72 -check-mpi ./miniFE.x verify_solution=1 nx=10

  • result:

    solution matches analytic solution to within 0.06 or better.
    [0] INFO: Error checking completed without finding any problems.
    

• openmp/src

  • gcc 11.2.0

  • Intel(R) MPI Library for Linux* OS, Version 2021.7

  • ITAC 2021.6.0

  • command: make clean && make -j && OMP_NUM_THREADS=10 mpiexec -n 2 -check-mpi ./miniFE.x verify_solution=1 nx=10

  • result:

    solution matches analytic solution to within 0.06 or better.
    [0] INFO: Error checking completed without finding any problems.
    

• mkl/src

  • icpc (ICC) 2021.6.0 20220226

  • Intel(R) MPI Library for Linux* OS, Version 2021.7

  • ITAC 2021.6.0

  • MKL 2022.1.0

  • command: make clean && make -j && mpiexec -n 72 -check-mpi ./miniFE.x verify_solution=1 nx=10

  • result:

    solution matches analytic solution to within 0.06 or better.
    [0] INFO: Error checking completed without finding any problems.
    

Let me know if you need more testing or don't see the changes fit.