Backward compatibility of pg.interpolate

[florian-wagner]

Before pg.interpolate was a high-level Python function, the following worked for a 2D mesh:

# interpolate potential to surface
x = np.linspace(emesh.xmin(), emesh.xmax(), 100)
pot_surface = pg.interpolate(emesh, pot, x=x, y=np.ones_like(x) * height)

now the z argument has to be given explicitly:

# interpolate potential to surface
x = np.linspace(emesh.xmin(), emesh.xmax(), 100)
pot_surface = pg.interpolate(emesh, pot, x=x, y=np.ones_like(x) * height, z=np.zeros_like(x))
```