Add option to save out raw intensity images to the .h5 file

[laureho]

Would it be useful for general users to also be able to save the raw data, not only the segmentation masks? e.g. This would allow the user to crop the raw intensity patches belonging to individual tracks and visualize them as a montage.

It would be identical code to the segmentation property and the write_segmentation method in the HDF5FileHandler, so should be easy to implement.

[quantumjot]

Maybe we could make a generic write_images function that we re-use for segmentation?