Changes

[Feature #60, #134] Extend JOPA API with enum mapping classes.
 (commit: 25c741b)

[Feature #60] Add a marker annotation for enums mapped to object property values (OWL ObjectOneOf).

Implement validation of such enums.
 (commit: e0ff261)

[Feature #60] Implement ObjectOneOfEnumConverter for object property enum values.
 (commit: 69d2bea)

[Feature #60] Create ObjectOneOfEnumConverter when resolving converter for enum-valued object property attribute.

Extend test metamodel with an enum-valued object property attribute.
 (commit: 02df0a4)

[Feature #60] Implement enum value mapping for singular object property attributes.
 (commit: 299e8b7)

[Feature #60] Implement enum value mapping for plural object property attributes.
 (commit: 80b6f18)

[Feature #60] Integration tests of mapping enum constants to ontological individuals.
 (commit: 3b21ae1)