Clearly define how access modes in data grant apply to the data in data registration

[elf-pavlik]

We should clarify if we will use Create, Update and Delete access mode. I think we should use Create if we want to allow creation of new data instances, which means currently SelectedInstances scope would not allow Create as valid access mode.

Having shape validation in place, distinction between Update and Delete also becomes practical. Resource can't be set to empty by Update since it would validate against the shape.