Added Normal Distribution

[rvchess]

All functions from https://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.norm.html
except the Expect function.

[coveralls]

Coverage remained the same at 100.0% when pulling 5d15362 on rvchess:rubens_suite into 5391ebe on montanaflynn:master.

[montanaflynn]

Thanks so much @rvchess! This looks like a wonderful addition to the library.

I have two small requests:

1. Remove .idea directory.
2. Use the convention of starting comments with the function name, from this link:

https://blog.golang.org/godoc-documenting-go-code

Notice this comment is a complete sentence that begins with the name of the element it describes. This important convention allows us to generate documentation in a variety of formats, from plain text to HTML to UNIX man pages, and makes it read better when tools truncate it for brevity, such as when they extract the first line or sentence.

So this:

// Probability density function.
func NormPdf(x float64, loc float64, scale float64) float64 {

Becomes:

// NormPdf is a probability density function.
func NormPdf(x float64, loc float64, scale float64) float64 {

[montanaflynn]

Everything looks good besides the question of compatibility.

[montanaflynn]

@rvchess thanks so much for your contribution to stats! Everything looks good so I'm going to merge this PR 👍

[montanaflynn]

@rvchess I just noticed there is an empty file called go being added. Can you remove that?

[rvchess]

It was fun! Unfortunately it seems that Go doesn't have any built in support for certain non elementary functions such as the hypergeometric, beta, etc. It also doesn't have much support for calculus operations which makes modeling other distributions pretty difficult. Enjoyed this one though.