Add popper workflow for comparing xlearn vs liblinear for the higgs demo

[nicolecastillo]

In this PR, we add a Popper workflow for automatically downloading and verifying the complete HIGGS data set from UCI (which has 11 million entries), running the benchmark to compare liblinear vs xlearn, and finally generating a report with a chart that shows the results including error bars.

Through this PR,

• We add a popper folder that includes all the scripts, the workflow file wf.yml and a README.md that explains how to run it.
• We also modified the README.rst from the Higgs demo to include information about the workflow using Popper.