Make rerun_demo default to the cube

[jleibs]

python -m rerun_demo             # runs cube demo
python -m rerun_demo --cube      # runs cube demo
python -m rerun_demo --colmap    # runs colmap demo

Screen shot for first run on a fresh app instance:

Because we're calling the colmap demo --colmap I also named the rrd file as such.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I've added a line to CHANGELOG.md (if this is a big enough change to warrant it)

[jleibs]

Downloaded wheel from artifacts and confirmed it has correct rrd file and runs as expected.

[nikolausWest]

Nice