Split models into separate repositories

[Florian R. Hölzlwimmer]

I think it would make sense at this point to split the repository into separate standalone repositories, similar to what conda forge does.
The kipoi/models repository then should contain the models as git sub-modules:

models/
  |- basenji      -> github.com/kipoi/kipoi-basenji <revision 1234aedf>
  |- framepool    -> github.com/kipoi/kipoi-framepool <revision 4327abdf>
  |- expecto      -> github.com/FunctionLab/expecto <revision 9a8b7c6d>
  |- [...]

Advantages:

• Direct versioning of the models
• Smaller main repository
• Allows per-model dependabot updates
• Technically, we could even have external model repositories (like expecto above)