Get distribution parameters from mean and std

[FriesischScott]

This PR provides convenience methods to get the parameters of a Distribution from its mean an standard deviation (if possible!).

Basic list of Distributions:

• [x] Beta
• [x] Gamma
• [x] Gumbel
• [x] Logistic
• [x] LogNormal
• [x] Rayleigh
• [x] Uniform

These are the ones we had in Cossan. I need to check which of the remaining UnivariateDistributions in Distributions.jl can be built from mean and standard deviation.

Closes #105.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (b921a03) 98.76% compared to head (fabe738) 98.79%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #116 +/- ## ========================================== + Coverage 98.76% 98.79% +0.03% ========================================== Files 25 26 +1 Lines 970 998 +28 ========================================== + Hits 958 986 +28 Misses 12 12 ```

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