PERCLASS_EXAMPLE_CALLBACK  Example function called when selecting
 sample in the scatter plot

 Callback is used by the sdscatter when providing it as extra argument:
 >> sdscatter(data,'callback',@perclass_example_callback)

 Clicking on a data sample will invoke this callback function. The
 callback function receives the data sample the user clicked on.

 This example callback prints sample label and, if present, sample
 identifier (unique numerical id that the user might have set up).

 Callback may, for example, load external image, based on a filename
 stored as sample meta-data and show it in an external figure.