[distributions] Hyper-Exponential: added more constructors and class construction test case.
Merge branch 'feature/hyperexponential_dist' of https://github.com/sguazt/math into hyper_exponential
Modify Marco's 1 and 2 arg constructors to use SFINAE to differentiate between iterators and ranges.
Add a 1 arg initializer list constructor.
Update tests accordingly.