Adding ipython notebook for still analysis

Old file just being added a bit late.  For analysis of unshaken
stationary video to track uncertainty over time of positions and
orientation.

	new file:   still_analysis.ipynb
Some small tweaks for positions.py

If closing size is 0, don't do it!

png plots shouldn't be saved in the data directory, save them wherever
the output file is saved, but go ahead and keep the original file names,
just change `tif` to `png`.

	modified:   positions.py