* Said generic can be replaced with an MPI-specific version, thus
  cleanly separating serialization of meta-data itself and MPI-
  communication.

* This results in an API change for CDI-pio initialization. Users are no longer
  required to specify the number of namespaces in advance.

* This way all header elements can later be accessed directly.

* This will later ease distinguishing both in the MPI window index.

* This is intended to later group all information for each window.