remove large files from git commit history

[ctb]

The whole repo history is in the ~100 MB range, while the current master is only a few MB. This makes it unnecessarily slow to check out the git repo :)

Using the command here, it looks like we need to go remove any trace of testing_out/ from the history. Not quite sure how to do that but this github help article looks legit.

[bluegenes]

apparently this requires all PR's to be closed, bc merging will be affected? Need to set a time to do this to avoid losing work.