Resolve reverse links using the forward property's path, not its Python name

[apdavison]

Fixes #92.

Currently, resolving or counting a reverse property (e.g. license.is_applied_to.resolve(client)) sends the same filter, keyed by a single Python name, to every target class. The forward property that makes the link does not always have the same Python name in each class: it is license in DatasetVersion but licenses in ModelVersion and SoftwareVersion. For such classes query generation raises ValueError: Invalid filter, so the whole reverse property cannot be resolved. On master this affects 17 (reverse property, target class) pairs in v4 and 14 in v5.

In addition, 72 reverse properties stand for several forward properties (e.g. BehavioralProtocol.used_in covers associated_protocols, behavioral_protocols and protocols), but only the first is queried, which fails for any target class that lacks it.

This PR adds KGQuery.from_reverse_property(), which identifies the forward properties by their path (the same in every class) rather than by Python name. It builds one filter for each matching forward property in each target class. resolve() and count() use these per-class filters, and resolve() returns each node once even when it links through more than one property (as ProtocolExecution can, via both behavioral_protocols and protocols, in v5). count() counts such a node once per property, as documented in its docstring.

It also removes the reverse-KGQuery block in KGObject.__init__, which never runs: KGNode.__init__ has already set every reverse property, and it passes a callback argument that KGQuery does not accept.

Tests:

• every reverse property in v4 and v5 gets a filter for each target class, and each filter generates a valid query
• the license/licenses case, the v5 two-property case, and de-duplication in resolve()
• a live test that CC-BY-4.0's is_applied_to.count() succeeds

Not covered here: reverse links whose targets are embedded classes (#82), and MRIScannerUsage, whose query generation fails for a separate reason.