Make isFirstEvent work as expected in wrapped processors

[tmadlener]

BEGINRELEASENOTES

• Fix an issue where isFirstEvent always returns false in wrapped processors by calling setFirstEvent after processing the first event.

ENDRELEASENOTES

This was discovered by Jenny and if not fixed, would mean that processors relying on this would need quite a bit of work, see e.g. iLCSoft/MarlinKinfitProcessors#19

• Needs Add accessor class for wrapper to allow private method access iLCSoft/Marlin#47

[tmadlener]

Updated the documentation to state that isFirstEvent does not work in mutli-threaded mode. Should I also add a test that checks that this is working as expected with a simple test processor?

[andresailer]

Merge this as well ?

[tmadlener]

Yes, unless we want a test for this?

[andresailer]

Having a test is always better of course. But can add one later, to have this in the tag.