Fix the rate parameter of the gamma distribution used to represent
the negative binomial distribution as Poisson mixture.

…ution

Make the unchecked sample methods of the Poisson distribution internal
instead of private.
Within the negative binomial sampling, call the Poisson sample method
instead of duplicating the implementation.

The negative binomial distribution is undefined for p = 0
(asymptotically, it reaches infinity, which cannot be represented as
integer). This also avoids unhandled divisions by 0 or other undefined
mathematical operations in the properties and an endless loop in the
sampling method.

…ution

Use correct XML encoding of the less than operator for the description
of the range of the p parameter.

…tribution

Adjust the unit tests after parameter restriction and proper handling
of edge cases:
- Use p = 0.1 instead of p = 0.0
- Add a test case with p = 0.0 in test for invalid parameters
- Add special case in probability validation for p = 1.0 and r = 0.0