Add a scheduler check to skip backward pass

Context

Modify the scheduler so that it checks every node for a skipBackward dynamic attributes. If a node has this attributes (and its value is true), then we do indeed skip backward.

Additionally, add this attributes to some nodes of the Leaky Metaoperator(this group of nodes constitutes the reset mechanism, and thus can be skip entirely for now, later on this should be modified).

Modified files

• src/scheduler/SequentialScheduler.cpp
• src/operator/MetaOperatorDefs/Leaky.cpp

Fix parts of #251 (closed)