Enhancements and bug fixes for event detection

Fixed improper termination bug in sdezero(). Added warning if unsupported NonNegative or DiagonalNoise options are used. Added better support for events in sde_euler() and sde_milstein(). Minor fixes and added warning about drift less diffusion in sde_ou(). Minor fixes in sde_gbm().

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