finished group-role assignment tests. get_roles() no longer throws errors when no assigned roles are found (it returns an empty DataFrame instead)

removed roleAssignments as an attribute of person/group. Roles can be listed with the .get_roles() method instead

pybis: removed redundant text form error message - allows for more flexibility when showing the error to the user

pybis: added linkedData support to DataSet; created GitDataSetUpdate; using parse_jackson in pybis.get_dataset

Jupyter related projects moved to git@sissource.ethz.ch:sispub/jupyter-openbis-integration.git + changes made to PyBis for the integration merged back

for predicate and descriptor fields - update documentation