Remove RegionOfInterest

This finishes work on #73 (closed), and goes already further to have a new ROI implementation.

Contained in this MR:

• Remove RegionOfInterest as a separate class
• redesign the realization of ROI functionality
• 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 (closed) ("Further consolidate ICoordSystem")

More docu regarding ROI is contained in the IDetector docu