added galaxy mass tutorial on quantities
added mc mass part
cleared output in Quantities
fix case on Quantitites.ipynb
Content tweaks in Quantities tutorial section 1
edits to section 2 of Quantities
Added section 3 example
Merge pull request #1 from eteq/Quantities

Enhancements to Quantity tutorial
fixing typos
Edit section 2
tweaks to some wording, mostly S2
Merge pull request #3 from jfoster17/Quantities

Edit section 2
metadata for Quantities
Merge pull request #4 from eteq/Quantities

tweaks to some wording, mostly section 2
community suggestions
astrofrog's comments; name change
name change
astrofrog's comments
Merge pull request #51 from abonaca/Quantities

Quantities tutorial