Refactor retrieval of EDM4hep event Frame into method

[tmadlener]

BEGINRELEASENOTES

• Refactor the retrieval of the EDM4hep event (Frame) into standalone method

ENDRELEASENOTES

Another standalone outcome of #236

As far as I can tell there is no way that the optional is not engaged that didn't already break before. The behavior before was

• If there is a PodioDataSvc, we get the event Frame from there
• Otherwise we get the Frame from where the IOSvc puts it
  • In case it is not there we create an empty local Frame for which the reference_wrapper will immediately dangle.

Hence, we could probably remove both the optional and the reference_wrapper (which I didn't do yet).

@jmcarcell anything I missed here about the previous behavior?

[tmadlener]

Playing around with godbolt, it indeed looks like the previous version only worked by accident (if it worked at all) (reproducer).

I have switched to returning a const & and throwing an exception in case there is no valid Frame available. At least for our tests everything seems to work.

[tmadlener]

Tests are passing for me locally. I have to investigate why they are failing here.

[tmadlener]

Not sure why tests were passing for me locally, I can't seem to reproduce this. I have added the necessary functionality to put an empty Frame into the TES if none is there already.