Degree increase/reduction should accept parameter times = 0, which does nothing

[carlos-adir]

At the version v1.0.8, calling degree_increase or degree_decrease raises a ValueError when the parameter times = 0 is given.
It should do nothing in that case.

Steps to reproduce:

from compmec.nurbs import Curve

knotvector = (0, 0, 0, 1, 1, 1)  # degree 2, npts 3
curve = Curve(knotvector)
curve.degree_increase(0)  # Raises ValueError
curve.degree_decrease(0)  # Raises ValueError
curve.degree = 2  # Does nothing since degree is already 2