Writer: Ignore objects that are not collections

[jmcarcell]

Currently if the Writer finds something that is not a collection either produced by the functional algorithms or the data handle-based algorithms it will not write any frames. After this PR, a warning will appear once and they will simply be ignored.

BEGINRELEASENOTES

• Writer: Ignore objects that are not collections in the store and write an output Frame
  • Add const where possible
  • Add [[maybe_unused]] to avoid a warning about not using the result of a .release() (the warning is correct in most cases, in this case it's the Gaudi store who owns it).
  • Remove some code in the Writer that is not necessary.

ENDRELEASENOTES

[jmcarcell]

If there aren't any comments I'll merge this today, this is necessary to make the Marlin wrapper work with the IOSvc.

[jmcarcell]

I think this is good to go. I was able to run the Marlin wrapper with IOSvc algorithms after this PR (will open PRs soon).