reading/writing of extra tags in quakeml

Writing "extra" information (https://github.com/megies/obspy/commit/ef8fd5f3cce#L0R1174) should be done for all quakeml elements (alle elements support custom tags after the usual quakeml tags). Further, this should also be done when reading quakeml (i.e. in `Unpickler()`).
In principle also custom attributes are allowed for all quakeml elements, but this is probably not important, when one can use a custom element instead.