EntityHDF5::group() now public & SectionHDF5::link using createLink

The new concept of using hard links in HDF5 via group::createLink has been implemented for SectionHDF5::link.
Made link setter check for existing link and remove it before set
Keep shared_pointer in SectionHDF5::link instead of de-ref
Merge pull request #330 from balint42/refs_by_links

Refs by links