Added an optional reference image to Galbally IQM

This modification allows to pass an optional reference image to Galbally IQM feature extractor. The default reference image is a smoothed version of the input image. Default behaviour doesn't change.

@onikisins: please assign a person to review this (e.g. @dgeissbuhler). This then pops-up as an e-mail to this person and a todo item.

assigned to @dgeissbuhler

@andre.anjos thank you! @dgeissbuhler can you please review and merge it if fine for you. Thanks!

merged

mentioned in commit 2effc8f4