* add triangle distribution and associated tests
* add efficiency, constraint checking, and code convention to triangle distribution
* change to follow Categorical structure exporting approach, add MarshalParameters function, and verify parameter constraints in UnmarshalParameters function
* change to export Triangle.Source and remove from NewTriangle signature, add checkTriangleParameters function, and fix bug
* fix comment, replace ln2 with math.Ln2, and add test for a=c
* fix to return float64 for integer division