Add support for Gamma Distribution

[hot9cups]

Go through the Gaussian distribution class, the Binomial class and the Exponential class which are subclasses of General distribution class.

I would like to expand this to include the Gamma Probability Distribution.
It should be able to calculate mean, standard deviation, and create basic plots like those in the classes I linked.

[rachitmanas]

I would like to work on this issue.

[The-Shwin]

I opened a PR for to solve this issue since it didn't seem work had already been done.