First step towards refactoring the instance class into a class that
describes the SN population and an instance class.
snUniverse.py: class which describes the properties of SNIa in the universe in terms of
a model, and in terms of the distribution functions the parameters of
that model take. It would be good to define an abstract class which does
this in a general case and put what I have now in a specific
SALT2-universe class which inherits from that class.

test_snUniverse.ipynb: an ipython notebook to test that this class is
working correctly.