Enhancements and fixes for Brownian bridge interpolation

Added eta parameter to scale noise in Brownian bridge.

Input checking in sde_interp() completely reworked. Still messy.

All unit tests pass for sde_interpq(). More and better needed. Unit tests for sde_interpqn() incomplete and not really working - sde_interpqn() needs work. sde_interp() needs units tests. Also need validation and benchmarking.

Signed-off-by: Andrew D. Horchler <adh9@case.edu>
Event detection in special functions, OU, GBM

Added support for simple detection of multiple zero-crossing events in sde_gbm() and sde_ou().

Also changed Events option to EventsFUN

Edited help.

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