* stat/all: reduce random size and test tolerance to decrease testing time.
We were generating a lot of random numbers, which is slow. Decrease the size of those random numbers, and in some cases increase the tolerance to compensate. In a couple cases, pull out code from testFullDist to allow for more fine-grained testing. This decrases:
distmat from 4.5s to 0.5s
distmv from 24.8s to 9s
distuv from 65.2s to 13s
samplemv from 2.8s to 1.2s
sampleuv from 3.5s to 2.1s
* 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
