Add a 'clean' parameter to the NIST_MDD().get_pairing() function, to delete the minutiae with no pairing information

'export_latent()', 'export_print()', 'export_latent_annotated()' and 'export_print_annotated()' : Check if the file have been written to disk, and return the value

Change multiples call to the 'self.get_ntype()' function with a simple call stored in a local variable (for performance issues)

Add the NISTf().merge() function, and overload the '+' operator for NISTf objects with the merge() function

Change the structure of the NISTf.get_width() and NISTf.get_height() function (preparation for future modification)

If no image is set in the NIST object, the NISTf.checkMinutiae() function can not operate: no check is done in this case.