[distributions] Hyper-Exponential: forget to 'normalize' probability in constructor receiving iterators as arguments.
[distributions] Hyper-Exponential: removed useless assertion (the check is already done in the 'check_dist' function.
[distributions] Hyper-Exponential: use the exponential distribution for PDF.
[distributions] Hyper-Exponential: Improved test unit to support long-double type.
[distributions] Hyper-Exponential: fixed test results according to Mathematica test output where in place of decimal numbers we use the corresponding fractions.
Merged from branch 'hyper_exponential'.
[distributions] Hyper-Exponential: use exponential's PDF in hyperexponential PDF.
[distributions] Hyper-Exponential: fixed test results according to Mathematica output when instead of decimal numbers we use fractions.
Fix tolerance, add a missing L suffix on test data.