Add unit tests for HTM operator

[EronWright]

Flink provides a unit test framework for operators - or at least an integration test using an embedded Flink instance.

• Use a mock Network.
• Enhance the existing test (HTMIntegrationTest) to validate the output.
• Test keyed streams.
• Test the 'encoder input function' code (GenerateEncoderInputFunction), which takes a user type (i.e. case class, POJO, tuple as described by the subclasses of CompositeType) and produces a map of input data for the multi-encoder. Test the various types of user types, and test the value conversion logic (e.g. int -> double for ScalarEncoder).