Imaging: emitting regions

[ThomasCeulemans]

For creating models to compare with observations, it would be nice to have some indication of where the majority of emitted radiation (per pixel) comes from. Practically, I need to output source functions and optical depth increments on rays. Note: this is already somewhat impractically implemented in model::compute_image_for_point.

The main implementation challenge would then be to create a new image type for this 4D data (pixel (x,y), position on ray (z)), frequency) and coherently add it to the rest of the codebase (TODO: contemplate whether it is better to create a new class or subclass the existing Image class).