Split planar detector

This MR is for #98 (closed) (step 1), in order to split the Detector to Detector as interface and PlanarDetector as the implementation of the Cartesian coordinate system.

The modules would not be changed much in this MR to avoid it too big.

The configuration of using PlanarDetector type would best be implemented in this branch, probably as

case(PlanarDetector) : std::shared_ptr<Detector> detector = std::make_shared<PlanarDetector> (detector_section) ;