[distributions] Added Hyper-Exponential distribution.
[distributions] Hyper-Exponential: solved 'unused parameter' warning in 'mode' function.
[distributions] Hyper-Exponential: registered tests and fixed some test-related issue.
[distributions] Hyper-exponential: preliminary version of documentation (still incomplete).
[distributions] Hyper-exponential: range arguments to contructor should be passed by const-ref.
[distributions] Hyper-exponential: increased tolerance (from 1e-3 to 1e-4).
pull out for loop body of rounding test for later test
add explicit test cases for rounding corner cases
fix rounding errors

 - the direct predecessor/successor for 0.5/-0.5 was formerly wrongly
   rounded to 1.0/-1.0 rather than 0.0

 - integral numbers of the format 1xxxxxx1 filling mantissa completely
   were formerly rounded to the next integral away from zero
[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.
Up precision of tests.
Fix some conceptual compiler errors in hyperexponential_distribution.
[distributions] Hyper-Exponential: use the exponential distribution for PDF.
Add hyperexponential_distribution to concept checks, fix for warnings/bugs.
Fix some more warnings and enable tests for all the test types.
Change test type list when there are no long double math functions.
[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.