Move config dump in n.meta to own key

[lkstrp]

Changes proposed in this Pull Request

The full snakemake config is currently saved in n.meta. This PR moves it instead to n.meta['snakemake_config']. I want to add additional metadata to the Network to use somewhere else, so having it's own key is more organised.

But this is breaking if people rely on it somewhere.

[fneum]

Probably wise, but I definitely have used n.meta for accessing snakemake configuration options in post-processing scripts in the past. It's only a mild annoyance. Just add a note in release notes.