Reorg dirs: disentangle Models from Views

See merge request !93

Refactor model: instrument, footprint and background items

See merge request !92

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

Reorg dirs: reorganise `MaterilaItemUtils`

See merge request !91

Reorg dirs: Move Error class from utils/GUIHelpers to Models/Error

See merge request !90

Remove RegionOfInterest

See merge request !88

* 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")

Refactor model: limits items

See merge request !87

Moved ExternalProperty files to Models

See merge request !86