Fix Freebase entity prefix validation

[appleweiping]

Fixes #181

Freebase IDs in the knowledge graph data use m. and g. prefixes. The current validation accepts f. and rejects g.

This updates get_relations and get_neighbors and adds unit tests for both entry points.

Validation

python -m unittest tests.test_knowledgegraph_entity_ids -v

The current std.json contains 274 m IDs and 3 g IDs, with no f IDs.