Remove potential deadlock in PNFFT
Description
Replace mpi_world%comm in a Barrier call by the communicator that is
passed when the plan is created. This avoids a potential deadlock which
might occur if the communicator passed to pnfft is not MPI_COMM_WORLD.
Checklist
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I have checked that my code follows the Octopus coding standards
Closes #187 (closed)
Edited by Sebastian Ohlmann