Decouple schemes from codebase and put in separate schemes repo

[glabbe]

Schemes should be decoupled from the codebase and put into a separate biohansel schemes public repo with version control. It would make it easier to add/update new schemes without having to update the biohansel codebase and update it everywhere (PyPI, Conda, Galaxy, etc). The latest version of a scheme could be used by default checking the schemes git repo, or a specific version (tag, release or commit) of a scheme could be specified. This should cause a lot less headaches with future dev of the tool and schemes.