MPI: replace adjacent `send`/`recv` pairs with a call to `sendrecv`

[slayoo]

replace the lines below with a call to MPI.sendrecv()

https://github.com/open-atmos/PyMPDATA-MPI/blob/b63f69e0e0bcdc1678dea74324b5f5cfd1fd673b/PyMPDATA_MPI/impl/boundary_condition_commons.py#L128-L129

See numba-mpi/numba-mpi#85