Add the possibility of starting from the Nth event instead of the first one

[jmcarcell]

BEGINRELEASENOTES

• Add the possibility of starting from the Nth event instead of the first one with the argument FirstEventEntry to IOSvc.
• Add a test that ignores some events.

ENDRELEASENOTES

Someone asked for this and it was possible before with PodioInput.

[tmadlener]

It can be quite useful for debugging.

[andresailer]

Could we add the relevant argument to the release notes entry?