Fix `TensorflowModel` gradients' labeling

Closes #14 (closed) (see that issue for details on what is being fixed here)

Tasks:

• update TensorflowModel.compute_batch_gradients to use variables' names to label the returned tensors
• consider updating TensorflowModel.apply_updates to run some verifications on inputs' names
• extend Model unit test suite to include a verification that gradients and weights share the same specifications