'Pop up a dialog window for input of a number.
Arguments: title is the title of the dialog window,
prompt is a text mostly describing what numerical information to input.
default: default value
minval: minimum value for imput
maxval: maximum value for input
The number input must be in the range minval .. maxval if these are
given. If not, a hint is issued and the dialog remains open for
correction. Return the number input.
If the dialog is canceled,  return None.
Example (for a TurtleScreen instance named screen):
>>> screen.numinput("Poker", "Your stakes:", 1000, minval=10, maxval=10000)'

def numinput(self, title, prompt, default=None, minval=None, maxval=None):

    return simpledialog.askfloat(title, prompt, initialvalue=default, minvalue=minval, maxvalue=maxval)
