Changes proposed in this pull request

Adding actions to download and then run petprep on downloaded test data. "Caching" data locally on self hosted runner.
Creates two new workflows:

• collect_data.yml
• run_petprep.yml

Collect data is the more important of the two as it's set up to be called by other actions to either acquire or check to see if test data is already present on the runner. Uses scripts/collect_test_data.py to retrieve said data. There's additional work to be done the ensure that downloaded data that's stored locally is in a "clean state" since it persists on disk. It might be best to make the data read only after download then copy over to the an actions working directory via the parent caller.

run_petprep.yml builds and runs petprep on bare metal on the data collected via collect_data.yml/collect_test_data.py and plays two parts:

1. to help improve collect_data.yml by checking if the data there is amenable to petprep
2. testing for curiosity's sake whether it's worth installing all the necessary dependencies to run petprep without a container

I'm anticipating significant changes to each one of these actions, but wanting to get them merged so that we have a baseline to improve upon. Currently these actions only run on a single self hosted runner and won't bump heads against anything else.

Documentation that should be reviewed