quicksafe: working on ionized impurity scattering

-> need to think about materials approach:
as each model (DD, scatering, etc) has its own properties,
the XML layout is not straightforward. begging the question
where to put the thus specialized xpath expressions.
it should also work with, for instance, ipd -> so the only realy
solution to me is to make viennamaterials part of viennamini,
and thus the different library kernels (ipd, pugixml, ..) can
implement a specific api for accessing our unique material setup.