Initial commit for distribution package. Contains Exponential, Laplace, ...

...Normal, and Uniform
DLogProbDX and DLogProbDParam are accurate function names but seem clunky. I understand they are seeking the derivative of the exponent in relation to the PDF X value, and distribution parameters respectively. I'm not sure if there would be a better name as DerivativeOfExponentWithRespectToX is even worse.