Well-defined configuration input format

[jsjason]

In our current prototxt format used for configuration files, num_input and num_output are both declared as required. However, some layers such as PoolingLayers do not require these parameters because of the nature of the layer itself. We can either make those fields optional and go through a derivation process for layers that do not explicitly receive the fields, or move the fields to specific layer configurations instead of leaving them as common layer configurations.