Add safe profile for use when combining ontologies

Use case: combine multiple linked ontologies, use RG to determine formal ancestor relationships; avoid accidentally inserting intra-ontology is-as. See: [OWL is not modular](https://docs.google.com/document/d/1ddxt-npeps3t1sXiP-xpMV-Q0KiXGwsXx467U6SR4JE/edit#)

This can be done by preprocessing using robot remove with `--axioms "equivalent disjoint annotation"` as well as Domain and Range axioms (can't figure out how to do that in robot). It would be convenient to have this as a "graph walker" profile in RG, to avoid preprocessing. This will yield correct n-hop edges while not overwriting vetted assertions from source ontologies.

This issue may subsume:

- #127

Start by reading issue #127 and the linked “OWL is not modular” document, then inspect the existing relation-graph profiles and graph-walking entry points. The issue names no files or tests; completion would require a profile that handles combined ontologies without inserting intra-ontology is-a edges and preserves vetted source assertions.

Written by the indexing model from the issue text.