Edited tutorial to mention a few more methods of Quantity objects.

Added examples of using .dimensionality, .to_base_units, and
.ito_base_units.

Thanks for creating this project -- I've just starting using it and already I find it incredibly useful!

I find the .dimensionality, .to_base_units, and .ito_base_units methods particularly useful.  I thought it would be nice to add a few lines mentioning them in the basic tutorial page, so other beginning users notice them.

Thanks again!