feat(#1532): master-part entity clustering; drop 'ERD' naming

Enclose each master and its parts in a nested, unlabeled entity cluster; part
labels drop the master prefix (`C`, not `B.C`) since the enclosure carries
membership. Master and its parts share a rank (horizontal = derivation, vertical
= containment), except a part that depends on a sibling part, which is left off
the rank so the intra-group chain descends.

Also rename the diagram test module test_erd.py -> test_diagram.py and its
identifiers, and scrub 'ERD' from a code comment: DataJoint diagrams are not
ERDs. Adds a test asserting the entity cluster and the prefix-dropped part label.

Author: dimitri-yatsenko