Shorter test-suite execution time

This release updates the test for the new experimental capability for training neural networks. The current unit tests verify convergence for training single-hidden-layer networks using gradient descent with updates averaged across mini-batches of input/output pairs. Future work will include verifying the training of deep neural networks and introducing stochastic gradient descent.

This release replaces the deleted 0.6.1 release. Relative to release 0.6.1, the current release provides

1. Nearly an order of magnitude reduction in execution time for the mini-batch training example and for the similar unit test,
2. A fix for the training example to invoke the correct object definition function for the involved trainable_engine_t object, and
3. A fix for the inference_engine_t type-bound procedure to_json() to eliminate an erroneous trailing comma that led to invalid JSON output for networks with a single hidden layer.

What's Changed

• Fix json output, actually shorten test-suite runtime by @rouson in #58

Full Changelog: 0.6.1...0.6.2