Fixed bug in calculation of full analytic solution of mean passage time.

Some instances of Lambda_S needed to be replaced with Lambda_U in the full analytic solution. In my derivation I had put the wrong bounds on one of the integrals.

Also made improvements so that full analytic solution supports Lambda_S all the way up to Inf.

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