[distributions] Hyper-Exponential: added test for special cases (i.e., the one discussed under the 'Related distributions' section of the doc).
Add some tests for the error handlers.
[distributions] Hyper-Exponential: changed return type of 'probabilities' and 'rates' from 'vector<...>' to 'vector<...> const\&' to allow users to do things like this: 'std::copy(he.probabilities().begin(), he.probabilities().end(), std::ostream_iterator<double>(std::cout, " "));'.
[distributions] Hyper-Exponential: added a note to clarify that the probabilities return by method 'probabilities' are normalized.
[distributions] Hyper-Exponential: output of Wolski's example changed to a floating-point precision of 2 (obtained with 'std::cout.precision(2)').
[distributions] Hyper-Exponential: added two more graphs (one with different probs and same rates, and another one with different number of phases but same mean).
[distributions] Hyper-Exponential: added two more graphs (one with different probs and same rates, and another one with different number of phases but same mean).
Merge branch 'feature/hyperexponential_dist' of https://github.com/sguazt/math into hyper_exponential
Doc rebuild.
Revert "[distributions] Hyper-Exponential: changed return type of 'probabilities' and 'rates' from 'vector<...>' to 'vector<...> const\&' to allow users to do things like this: 'std::copy(he.probabilities().begin(), he.probabilities().end(), std::ostream_iterator<double>(std::cout, " "));'."

This reverts commit a8efc9a6372869cef1b1ec8861d69c30dcd9f19b.