Update use of GetLayer to new registry function

According to @Yangqing's note of September 26 at https://github.com/BVLC/caffe/blob/dev/include/caffe/layer_factory.hpp#L119, `GetLayer` is obsolete; however we still use it during net construction.

This patch replaces `GetLayer` with the newer `LayerRegistry::CreateLayer`. It seems to me this is the only place we are still using `GetLayer`, so maybe it should be removed altogether as well?