Introduce SessionItem::setGroupPropertyType, a templated version of SessionItem::setGroupProperty, to reduce the need for casting

* refactor IDetector to have built-in, n-dim ROI support
* add methods for implicit ROI handling (no distinction to be made on caller's side)
* add docu for ROI
* refactor CoordSystem classes to not use detector and its ROI related things internally,
  but at caller's side. This eases transition handled in issue #71 ("Further consolidate ICoordSystem")

Make BeamDistributionItem::setDistributionGroupItem a template to reduce the need for casting and improve type safety