We agreed that `gnpy.core` should only contain stuff for propagating
wavelengths. Conceptually, JSON parsing and even instantiating these
network elements from data obtained through JSON is *not* something that
is on the same level -- and this will become more important when we move
into YANG format in future.
Also, instead of former `gnpy.core.equipment.common`, use
`gnpy.tools.json_io._JsonThing`. It is not really an awesome name :),
but I think it sucks less than a thing called "common" which would be no
really longer any "common" in that new file.
Change-Id: Ifd85ea4423d418c14c8fae3d5054c5cb5638d283
