Minor additions

New Feature

• The possibility to subs symbols with gduals is added to python:

from pyaudi import gdual_double as gdual
x = gdual(1, "x", 4)
x = x**2
y = gdual(0., "y", 1)
newx = x.subs("dx", y)
newx == gdual(1, "y", 4)*gdual(1, "y", 4)
newx.order == 4