Add a script to compile a list of all requirements for Binder

[adrn]

After merging #617, we now have the tutorial requirements stored within each tutorial repo and globally within binder/requirements.txt. We should probably auto-generate the binder (global) requirements file so we don't have to track these things twice. Or we should change the organization of these things (see: #622) so each tutorial is in its own repo and they have truly independent requirements.