A dedicated library for statistical distributions

[astamm]

Layout of a statistical distribution library in Anima.
To be included in the library, a statistical distribution must inherit from BaseDistribution class and, as such, needs to have the following methods implemented:

• BelongsToSupport(x): checks if a value x is in the support of the distribution;
• GetDensity(x): computes the density at x;
• GetLogDensity(x): computes the log-density at x;
• Fit(sample): estimate the parameters of the distribution using the provided sample;
• Random(sample): draws sample from the distribution.

Currently implemented distributions are: Gamma distribution for real positive scalars, Watson distribution for axially symmetric vectors on the 2-sphere and Dirichlet distribution on the simplex.