completly refactored ViennaMaterials: new generic approach available

User can now setup data-specific access functors and forward it
to ViennaMaterials.
For instance, (as shown in the example) for pugixml and xml,
accessors can be setup, reflecting a speciifc xml markup.
--> this allows to support different xml markups without
touching ViennaMaterials at all -> the new approach should also
(in theory) support different backends, such as IPD, not tested though.