Require label of Node to be given

[p-snft]

We use labels for hashing of Nodes and also save them to a dict where the labels are used as keys. Thus, we implicitly assume that labels are constant. In my opinion, we should drop the default "None" from the labels, encouraging users to set them. Also, the label setter should be removed, as it is incompatible to the new dictionary approach.

[p-snft]

I consider this a bugfix, as energy_system["label"].label == "label" can be false at the moment, which should not be the case.

[henhuy]

Sounds reasonable. And changes LGTM.
Needs many adaptions in current oemof.solph environments and tests though.
Breaking change for oemof.solph

[p-snft]

Sounds reasonable. And changes LGTM. Needs many adaptions in current oemof.solph environments and tests though. Breaking change for oemof.solph

Actually, this does not need any changes besides those because of #40. Explicitly not setting the label (label=None) still works, so not setting a label is still allowed when interfacing with solph. Only, changing a label is no longer possible - but as I said, it should not be as it can cause inconsistencies.