[rvg] Hyperexponential: removed the 'unnamed' namespace and improved (hopefully) the 'check_probabilities' function.
[rvg] Hyperexponential: improved 'normalize' method.
[rvg] Hyperexponential: improved the 'normalize' function and changed the name of 'normalize' function to 'normalize_copy' for the overloaded version with 'const' parameter.