DSI spec proposal: change criterion for unmangled succession to only use positive integers in edition number tuples

[castedo]

This proposal is to replace the following two criteria in DSI edition 2.2 (1wFGhvmv8XZfPx0O5Hya2e9AyXo/2.2/):

Criterion: Edition numbers are non-empty tuples of non-negative integers.

Criterion: The final integer of every snapshot edition number is positive.

with this single criterion:

Criterion: Edition numbers are non-empty tuples of positive integers.

Rationale:

See #4

Zeros are handled in a special way on popgen.es and in Hidos, but after using this features for a while, I'm starting to question it's utility.

Acceptable baseline interoperability is to ignore these "unlisted edition" with zeros in the edition number tuple. For now, this "unlisted editions" can be demoted into an experimental feature and the DSI spec can just focus on the simpler level of interoperability which is to consider a succession with zeros a mangled succession that can be ignored.