WIP: Add notebook demonstrating tilt calibration.

[danielballan]

Currently, this notebook gets as far as plotting intensity vs. q. Ultimately it should:

1. Get as far as obtaining the tilt angles
2. Include the necessary image file and associated data (wavelength, etc.)
3. Offer more explanatory text
4. Demonstrate the convenience function currently sitting in a PR (radial integration: powder data scikit-beam#236) as well as this more illustrative step-by-step approach

[ericdill]

Presumably you'll want to commit one or more image files?

[danielballan]

See point 2 above.

[ericdill]

ah, very good... ignore me