Property-value redesign

Hi all, this is the redesign that store properties as datasets instead of groups. 
As said in  issue #220 it looks much better in the hdf  file. I, for now, decided to not support deleting of values.  
Since this pull request would introduce some changes to the API (e.g. Section::createProperty) it would be time for a tag before possible merge, isn't it?!

Excuse the merge commits due to  rebasing, etc... I gave up to clean that...
If I am understanding this correctly, the `property_group` is now more like a `section_group`.