* Make Relay Keras frontend support networks created using
Tensorflow (1.13) Keras implementation (tf.Keras)
* Modify Keras frontend tests to run from a class rather than a
function based script
* Adjust Keras frontend tests to run with both 'Keras' and 'tf.Keras'
* Change "TestKeras.test_forward_merge" to validate instances by
class name rather than instance type
