Changes

[Enhancement #73] Refactor OntoDriver API descriptors in preparation for supporting multiple contexts per assertion/subject.

AxiomValueDescriptor has been split from AxiomDescriptor, as it can contain only one context per assertion/subject to prevent ambiguity issues when saving values.
 (commit: 91dc0cf)

[Enhancement #73] Ensure consistent handling of contexts in OntoDriver axiom descriptors.
 (commit: 78975d1)

[Enhancement #73] Refactor Sesame driver to support the new plural contexts supporting axiom descriptors.
 (commit: 3753749)

[Enhancement #73] Test support for multiple read contexts in Sesame OntoDriver.
 (commit: b5c8b13)

[Enhancement #73] Refactor Jena OntoDriver to support the new axiom descriptors with plural contexts.
 (commit: e796dd6)

[Enhancement #73] Testing support for using multiple contexts in assertion loading in Jena Driver.
 (commit: 80ec625)

[Enhancement #73] Fix issues with AxiomDescriptorFactoryTest.

Also put Hamcrest into parent pom to make it accessible in tests in all modules.
 (commit: b8b6fd2)

[Enhancement #73] Refactor types OntoDriver API to support multiple contexts. Fix test issues.
 (commit: 768e089)

[Enhancement #73] Refactor descriptors to support multiple contexts per descriptor and attribute.

The changes are as small as possible w.r.t. existing API.
 (commit: f015826)

[Enhancement #73] Add tests for the updated descriptor behavior, fix issues.
 (commit: 69ef048)

[Enhancement #73] Adapting JOPA impl to the new descriptors API. WIP.
 (commit: bb4baa1)

[Enhancement #73] Adapting OOM in JOPA to the new descriptor API. Extend Descriptor with methods for getting single context (used for write operations).
 (commit: 010c219)

[Enhancement #73] Refactor OntoDriver API (and implementations) to support contains call with plural contexts.
 (commit: a4abba8)

[Enhancement #73] Use OntoDriver plural context contains API in JOPA.
 (commit: f89e020)

[Enhancement #73] Working on modifications of second level cache to support plural contexts.
 (commit: 21d1c0e)

[Enhancement #73] Update second level cache to work with plural contexts.
 (commit: f007e55)

[Enhancement #73] Fix tests after context-related refactoring (WIP).
 (commit: fe50d47)

[Enhancement #73] Fix integration tests after plural context changes.
 (commit: e1bac05)

[Enhancement #73] Add integration tests for plural contexts support.
 (commit: 02285dd)

[Upd] Update dependencies (RDF4J 3.4.2, JaCoCo plugin 0.8.6).
 (commit: 22bd079)

[Upd] Update to RDF4J 3.4.3.
 (commit: 5cd44b9)

[0.16.0] Bump version and update release notes.
 (commit: 9fc6828)