Initialize SysDecorationHanles to ensure we can write out the branches all the time

After quite lengthy discussion today, we realised that we need a solution to have all aux names registered by the end of first event. This does not work for SysDecorationHandles as accessors are created on demand.

This MR adds initialize which accepts a list of systematics and creates the needed accessors. There are two main changes in this MR:

• A separate addAffectingSystematics function is added to setup the decorators if needed.
• A newly added initialize is mandatory.

/cc @akraszna @krumnack @jburr