Pulled GremlinPipeline bits from tests into src
  - A DiagramMetric is now a voting metric, this means that each DiagramMetric has a Jena and Gremlin implementation.
Updated plugins-axiomatic to the new Optional<String> return type for the metric.
Updated apimetrics, structural and profiles to the new Optional<String> API.
Pulled test resoources -D definition into the diagrammetrics pom.xml
Dropped Guava Optional in favour of Java 1.8 java.util.Optional
Parsers now share a similar API, which should be refactored to a superclass
  - removed most of the tests in order to focus on making OWLAPI queries work.
Added missing files that
  (a) implement an OWL API Query, and
  (b) test that AtomSubsetAtom works.