moved PropertyAssignments to file property_assignment.py; added server_information to openbis object on login; moved class PhyiscalData to dataset.py

fixed long-searched-for-bug in _repr_html_. It was not there, but in the __getattr__ method, which returned empty string...

sample_type.get_semantic_annotations() now returns an empty DataFrame with the correct column headers

get_semantic_annotations() returns an empty DataFrame instead of a cryptic error message, when no annotations are found