Added some more prior distributions
This PR adds more prior distribution functions:
- Half-Gaussian
- Student's t
- Cauchy
- Exponential
- Beta
- Logistic
- Log-normal
- Gamma
- ChiSquared
It also adds classes that are synonyms for Gaussian distributions using the name Normal.
Where it is simpler to do so I just use the scipy.stats distributions and use the pdf, logpdf and ppf functions.
Edited by Matthew Pitkin