Fade synthetic objects

The generate_blob and generate_collection functions now take in an argument object_shape that can be either cylinder or tube, which automatically computes the parameters of the fade_mask function such that the blobs will have the desired shape. Examples have been added to the documentation for both the generate_blob and the generate_collection function.

Also, the fade_mask function has been modified to normalize the volume after fading in order to retain the original maximum value of the volume.