Create a popper compliant submodule with experiments

[carlosmalt]

I'd like to use this project as a "textbook example" for using Popper in our lab, for class projects, and for GSoC. Also, I've been trying to gain some headway (see popperized branch) but ran into some roadblocks. I blame a combination of my ignorance of containerization concepts (probably not that unusual among potential users) and missing documentation/tutorials.

[carlosmalt]

This was surprisingly hard:

1. I had to figure out that complex error messages meant that I simply wasn't running Docker
2. I wasn't sure at all how to run a Python script -- "Getting Started" only showed examples of shell scripts. It took me a while to discover popper search for finding appropriate actions.
3. I had to figure out that popper run was running main.workflow by default but if you provide that as an argument, it interpreted that argument as an action
4. I had to figure out how to correctly set paths in actions so that Docker images generated from these actions would work correctly (Thank you, @ivotron )
5. I had to understand how virtual environments for python work (virtualenv, python -m venv, and pyenv)
6. I had to find out that my actions failed because I put the repository in a Google Drive File Stream directory! These file sharing apps really mess with shell environments and name spaces -- both pretty essential for constructing popper workflows. (Thank you, @ivotron )

I filed a number of issues at Popper to help improve Popper's "Getting Started" documentation. 'Cause Popper's pretty cool once it works!