More Julian Code

[April-Hannah-Lena]

• multiple dispatch for cover
• split algorithms into multiple files
• add stdlibs to [compat] entry in Project.toml
• bugfixes

[gaioguy]

Will you add the function for computing marginals here?

[April-Hannah-Lena]

I previously committed the code but am holding it to make this PR a bit smaller and nicer. If you want, I can add the code here or on its own.

[gaioguy]

That's fine, just wondering. This one looks ready for merging then.