list downsampling methods in docstring of to_multiscale

[giovp]

This PR improves docstrings to explicitly list downsampling methods used to build the multiscale spatial image. This is the result.

    method : multiscale_spatial_image.Methods, optional
        Method to reduce the input image. Available methods are the following:
    
        - `'xarray_coarsen'` - Use xarray coarsen to downsample the image.
        - `'itk_bin_shrink'` - Use ITK BinShrinkImageFilter to downsample the image.
        - `'itk_gaussian'` - Use ITK GaussianImageFilter to downsample the image.
        - `'itk_label_gaussian'` - Use ITK LabelGaussianImageFilter to downsample the image.
        - `'dask_image_gaussian'` - Use dask-image gaussian_filter to downsample the image.
        - `'dask_image_mode'` - Use dask-image mode_filter to downsample the image.
        - `'dask_image_nearest'` - Use dask-image zoom to downsample the image.

thought it'd be a useful feature for users, please disregard if not in scope.

[thewtex]

Cool, thanks!

[thewtex]

neat!