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* Also replace calls of xwarning with Warning.

* This also adds separate documentation, header, Fortran wrappers and
  pkg-config file for the cdi-pio part of the library

* Introduces a specialization that fulfills the old interface.

* It is allowed to be NULL when no data is passed on a process.

* Since we don't use MPI_Put this might improve performance.

* This will enable functions in pio_interface later to be written
  without regard for header specifics.

* This allows to have the offset entry available for all types.