Allow ability to ignore the index error and return NaN in case the user...

Release notes

• Introduced a new flag to TQIParticleFourVectorObservable called mIgnoreError

Details

• When the index is > than the container size, an error is printed via ERRORclass. However, I've found times when I would've liked to print variables even if they may not be available in the event.
  • Example: Checking the leading/subleading b-jet pT at the base cut to understand the kinematics even though we don't apply the 2-bjet cut until later in the cutflow.
  • To ignore the errors, the syntax is to add :ignore_errors at the end of the observable expression. An example using pt():
    [P4:$(BJetContainer)[0].pt():ignore_errors]