Draft: Switch to mpi_f08

Description

Switch to mpi_f08.

Supported implementations:

• OpenMPI >= 1.7 [2013]
  https://github.com/open-mpi/ompi/blob/v1.7.0/NEWS#L65
• MPICH >= 3.1.1 [2014]
  https://github.com/pmodels/mpich/blob/v3.1.1/CHANGES#L10-L11
• Intel MPI >= 5.1 (needs Intel Fortran Compiler >= 16.0, part of Intel Parallel Studio XE 2016) [2015]
  https://www.intel.com/content/dam/develop/external/us/en/documents/intelmpi-5-1-update3-build223-releasenotes-linux.pdf (Ctrl+F “Fortran 2008”)

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.