Up precision of tests.
Fix some conceptual compiler errors in hyperexponential_distribution.
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: fixed test results according to Mathematica test output where in place of decimal numbers we use the corresponding fractions.
Merged from branch 'hyper_exponential'.