Make FlavorTagNNConfig less buggy

This is a minor rewrite of some configuration code around flavor tagging NNs. As @jabarr noticed a few days ago, the remapping options were accepted but then overwritten by local hacks. I'm telling the user configured remapping to take precedence over any local changes.

While I was in there I removed a lot of other similar gotchas, we did quite a lot of mutating the kwargs which was bound to confuse people in the future.