Add a function to export Selene-trained models to Kipoi

[kathyxchen]

We would like to add a method to the API (that can be used through Selene's CLI) that can help user create a model directory formatted correctly for export to Kipoi.

Currently, I have a PR #61 that shows/somewhat automates the steps I have taken to prepare a model we trained in case study 2 of the manuscript for export (just as an example). This is a precursor to the export function, which will be the focus of future work on this issue.

Many thanks to @avsecz for the ongoing help and discussion!