Create initial xri_etl script

[rminsil]

Overview

Create the initial script defined by the parent issue: #472

The parent issue has most of the details around how the script will work, so I won't repeat that all here.

The scope for this issue is just a basic POC script that produces the *.all.txt and *.(train/val/test).txt files. More complex things like data transformation will be left for later tickets.

[rminsil]

I accidentally closed this.
I intended to put a few PR's under this issue.

A new one is here: #491

[rminsil]

Another PR: #493