Simplify class relationships

[rouson]

After the recent replacement of trainable_engine_t's polymorphic differentiable_activation_strategy component by a non-polymorphic activation_t component, there is no longer a compelling motivation for trainable_engine_t to be completely separate from inference_engine_t. This PR replaces trainable_engine_t with a new trainable_network_t that extends inference_engine_t and is defined in the same module as inference_engine_t to give it direct access to the parent inference_engine_t's components.