[pycaffe] allow layers to have names different from their first tops

[pycaffe] allow layers to have names different from their first tops

Previously, net spec only allowed names to be assigned to Tops, giving
layers the names of their first tops. Now, names can be assigned to
Functions, which become layer names in serialization. Unnamed Functions
still get named after their first top, if present, or autogenerated, if
not. (This will allow top-less layers in a natural way.)