Bugfix release v0.3.1

Bugs addressed:

• Update networkx to version 2 (#6)
• Fix memory logger on windows (#5)
• Fix permission handling in cachable decorator (#9)
• Use yaml.safe_load in snakemake (#12)

Thanks to @fneum, @euronion, @FabianHofmann for issues and pull requests.

v0.3

• Switch to PROJ6