Quantities

Tutorial demonstrating the use of `astropy.quantities` in three parts:
1. Estimating mass of a galaxy (basic usage)
2. Estimating mass of a molecular cloud (advanced features)
3. Using Quantities with functions

[edit from @adrn:]
For convenience, here's a link to the rendered notebook:
http://nbviewer.ipython.org/urls/raw.githubusercontent.com/abonaca/astropy-tutorials/Quantities/tutorials/Quantities/Quantities.ipynb?create=1