Allow DL-syntax rules in HOWL files

[alanruttenberg]

Several of the reasoners allow inclusion of (a subset of) SWRL rules, implemented as DL-safe rules. It would be very nice to be able to include such rules in the body of an ontology. There is an abstract syntax for them and an RDF mapping and OWLAPI support. Example implementation in LSW: https://github.com/alanruttenberg/lsw2/blob/owlapiv4/owl2/swrl-rdf-mapping.lisp.
Example of use near the bottom. In that implementation I didn't use the OWLAPI but rather generated the RDF directly, but that's a minor detail.

[jamesaoverton]

We're working toward this with item 14 in the new design doc.