Add a reset_roots monad to event management

Signed-off-by: Christoph Höger <christoph.hoeger@tu-berlin.de>
Complete event_roots monad

Signed-off-by: Christoph Höger <christoph.hoeger@tu-berlin.de>
Add event root function

Signed-off-by: Christoph Höger <christoph.hoeger@tu-berlin.de>
Completed events interface

Signed-off-by: Christoph Höger <christoph.hoeger@tu-berlin.de>