Mean passage time function clean up -algorithm moved to private function

New private function meanpassagetime() containing guts of algorithm. No input checking.

Signed-off-by: Andrew D. Horchler <adh9@case.edu>
New soneholmesstat to calculate mean and variance of distribution

Uses meanpassagetime() private function and new varpassagetime() private function.

Signed-off-by: Andrew D. Horchler <adh9@case.edu>
New functions for variance and coefficient of variation for Stone-Holmes samples

Special code with input checking, handling of NaNs and values outside support of distribution, and approximate unbiased estimation for small sample counts.

Signed-off-by: Andrew D. Horchler <adh9@case.edu>