Add support for X-ray images and 2D annotations

[alexzwanenburg]

X-ray images are 2D projections based on attenuation of X-rays along the beam line. Their processing is particularly relevant for deep learning applications.

• Identify a dataset of X-rays with associated annotations.
• Ensure that for 2D imaging, the by_slice parameter is fixed to True.

... to add

[alexzwanenburg]

Implemented.