Much improved incomplete gamma function gammincf and updated private mean passage time function

gammincNegative has been replaced with gammincf, which is a complete re-write. Accuracy is greatly improved via the use of symbolic functions. Speed was improved by using the map and zip MuPAD functions to vectorize within the symbolic engine. A regularized incomplete gamma function capability has been added. The new private utility functions float2str and sym2float are used. The help was re-written.

meanpassagetime was updated to use gammincf - the regularized incomplete gamma capability in particular. The summation vector is now pre-allocated based on input sizes instead of assuming a column vector.

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