Implemented removal of links upon deletion of Sections
Implemented refs as links for EntityWithSources, corrections
Updated test cases: test if links are removed with entities linked to
Simplified deleteSource & implemented util::toId (get id from entity)
Simplified deleteSection
Implemented recursive deleteSection & deleteSource
Added test for deleteSource
Extended tests for testSection to check recursive deleteSection
Extended tests & fixed bug in recursive delete
Modified Group.cpp to check index before calling H5Lget_name_by_idx
Implemented refs as links in SimpleTag, fixed nasty bug
Modified testSimpleTag:test if all refs removed,fixed deleteDataArray
Implemented refs as links for DataTag in analogy to SimpleTag

Modified testDataTag too
Implemented refs as links for FeatureHDF5::data
Made Feature throw error if data is missing
Implemented refs as links for DataTag::positions

Adapted tests too
Removed none-deleter for SimpleTag::positions which is now obligatory
Implemented refs as links for DataTag::extents & removed ReferenceList

ReferenceList is gone for good from nix
Adapted tests too
Deleted removed ReferenceList
Modified "toId" to be a function instead of a functional