Implement the capability to read xyz files

The autoradiography scanner is saving the image data using its own native format (extension xyz).

This binary format is defined as followed.

1. The first four bytes are an header containing the height and the width as two 16bit integers with little endian encoding.
2. The remaining part of the data are the pixel values. Each of them is a 16bit integer in little endian encoding.

If an image is 50 x 25 pixels, then the total file size must be 4 + (50 * 25 * 2) bytes.

Implementation plan:

1. Allow the user to open xyz files along with tiff files from the file menu, implementing a custom binary decoder.
2. Change the file filter in the file open action so that the default filter is 'Compatible files (*.tif, *.xyz)', then standard tif and finally xyz files
3. Assure that the file drag and dropping is accepting also xyz files.
4. Assure that the file loading architecture is following the image_loader implementation irrespectively of the file format.
5. Assure that the save and restore workspace operations are not affected by this new feature.
6. Allow the user to save in xyz format as well. Extend the save_services for this purpose.
7. Update the documentation explaining this new feature. In particular the README.md, index.rst and usage.rst. Update the docstrings in the modified modules, if needed.