Define constraints on membership weights

[matejpavlovic]

I thought about that a bit, and realized that I don't see any fundamental reason to disallow any set of weights. The only exception is all weights in a membership being zero, which we probably do need to treat as a special case. I see 2 options here:

1. Declare such a membership invalid and complain / ignore it / crash / ...
2. Treat it as a special case and pretend that all weights are 1. I can imagine this being convenient for implementations that simply don't want to deal with any vote weights - it automatically falls back into counting nodes. However, it introduces additional complexity not just in code, but especially in the semantics.

Originally posted by @matejpavlovic in #459 (comment)