Use descriptors to shorten property code

[ketch]

We could use descriptors, as explained here:

http://nbviewer.ipython.org/urls/gist.github.com/ChrisBeaumont/5758381/raw/descriptor_writeup.ipynb

to simplify some code in solution.py, geometry.py, and perhaps elsewhere.

[mandli]

I agree, it would really make this code look nicer and be easier to read.