Added some logic to xml:

 - DIRQAImage holds data for one image
 - All DIRQA images are held in vtkMRMLRegistrationQualityInputNode
 - DIRQAImage can read itself from xml
Added further functionality to DIRQAImage

 - DIRQAImage can read associated image from disk
 - xml-file read is still hard-coded
 - file is read and first image is loaded when executing False Color