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.

Change the initial type of the list to AnnotationList(), and use the 'AnnotationList().set_format()' function