The initialization is split into a generic one placed in Process and a specific
one kept in GroundwaterFlowProcess.

Add a fwd include file, and a explicit template
instantiation definition in own compilation unit.

This decouples ogs.cpp from GWF process.

The result is written into a new or an existing
mesh's PropertyVector.

Mesh object is unfortunately modified, therefor the
const specifier is omited.

Using VtuInterface to write the mesh, which will
hold the resulting data in a PropertyVector.

Remove writing to an output stream.

Element variables are not yet used.