Refactoring / restructuring the Output class.

This is the first of a series of MRs that restructures the output facilities of ogs6.

This MR extracts Vtk and XDMF/HDF5 related functionality out of Output to a separate classes OutputVTKFormat and OutputXDMFHDF5Format that are derived from a common base class OutputFile. Furthermore some properties of the class Output are moved to the class OutputDataSpecification.

Next steps: Implementation of multiple Output objects.

@TobiasMeisel Thanks for discussion of the new structure.

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behavior change was documented?