No units selectable if the CV doesn't have the measured quantity unit relationships

Currently when we select a measured quantity for that there are no measuredquantity unit relationships in the CV, we can't select any unit in the SMS frontend.

While it is nice to have the pre-selection, it would be better if we would still allow the user to select the unit that makes sense for him/her.

So, if we have no measuredquantity unit relationships in the CV, we should allow to select from the whole list. If we have those relationships, those entries should be the first one as suggestions (as we have for the select boxes in the datastream linking) - still allowing all the others too.

And this behaviour should be taken for the accuracy unit & the resolution unit too (to have the suggestions).