Support for nonparametric sources and a bit of cleanup
Fixed pmf_test.py
Probabilities can be 0 in a PMF
Fixed the test for probabilities equal to zero
Removed the .mfd object from nonparametric sources
Merge pull request #228 from gem/nonparametric

Support for nonparametric sources and a bit of cleanup
added class method to construct planar surfaces from corner points
Merge pull request #229 from monellid/planar-surface-constructor

added class method to construct planar surfaces from corner points