Stone-Holmes Distribution functions now support finite Lambda_S

All Stone-Holmes functions updated. Previously, all but the two passage time functions assumed that Lambda_S=Inf. Now, all function take a Lambda_S argument, which may be equal to Inf (except in a few ill-conditioned cases). Some equations were re-written in order to support Lambda_S=Inf. Checks for Lambda_U < Lambda_S added.

The likelihood equations in stoneholmesfit() and stoneholmeslike() were completely re-written.

Examples in help were improved and new examples were added for stoneholmesmedian() and stoneholmesmode().

stoneholmesinv() and stoneholmesrnd() also updated for fix to erfcinv() that I discovered and reported. Mathworks fixed bug in Matlab 8.0.

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