Reconfigure to use binder branch

[rabernat]

Back in https://github.com/cgentemann/osm2020tutorial, @scottyhq set up this binder to use a separate binder branch, for reasons described here:
https://discourse.jupyter.org/t/tip-embed-custom-github-content-in-a-binder-link-with-nbgitpuller/922

The binder was launched with a link like https://binder.pangeo.io/v2/gh/cgentemann/osm2020tutorial/binder?urlpath=git-pull?repo=https://github.com/cgentemann/osm2020tutorial%26amp%3Bbranch=master%26amp%3Burlpath=lab/tree/osm2020tutorial/%3Fautodecode

To re-implement this here, we would need to:

• re-create a binder branch and put the binder config in it
• remove the binder directory from the master branch.

Question: will this work with the start script?