Pixel detector class

This MR proposes the following changes to the detector model tree:

1. Make the base DetectorModel class abstract:
   • Remove parts of the ctor that handle pixel-specific configuration keys (e.g. number_of_pixels). The ctor now only looks for model-inspecific parameters such as type, sensor_thickness or excess dimensions and support structure parameters.
   • Remove virtual specifier from methods which are not overridden in specialized detector models.
   • Make virtual methods purely virtual.
2. Introduce PixelDetectorModel class which inherits from DetectorModel and handles the pixel-specific parameters. This is basically the original DetectorModel implementation.
3. Specialized pixel models HybridPixelDetectorModel and MonolithicPixelDetectorModel now inherit from PixelDetectorModel instead of the base class.

(These changes result from discussion on !528 (closed) and were initially proposed in !531 (closed) which was closed due to rebasing issues)