Expose Net.copy_trained_layers_from in pycaffe

This allows finetuning from Python, for example.

Misgivings: `copy_trained_layers_from` is a very long name, while pycaffe mostly has pretty succinct naming. Also, the layers don't really have to be trained at all, so maybe a simple `copy_from` would be better. But this is what it's called in the C++ interface.