stats::probs::cdf improve docs (#316)
stats::probs::cdf improve docs (#316)

* Initial attempt at doctests for cdf. Compiling, but the test case is not there, yet.

* Moved doctest to module level documentation. Not yet finished with examples for all.

* Doctests for almost all CDF functions should be there. CDF and Entry structs also documented.
rust-bio/rust-bio