Fix opening files on the command line.
Fix opening files on the command line.

The check for an existing molecule isn't needed when opening a file.

Change-Id: Ic92f2b4c3d6b916a1fb4e65bdc9394c15de30eae

wadejong/avogadroapp,ghutchis/avogadroapp,ghutchis/avogadroapp,ghutchis/avogadroapp,wadejong/avogadroapp,OpenChemistry/avogadroapp,OpenChemistry/avogadroapp,OpenChemistry/avogadroapp,ghutchis/avogadroapp,OpenChemistry/avogadroapp,wadejong/avogadroapp