Generates a sphere-like surface with unit radius centered at the origin.
Parameters :
d: int (default: 10) :
Level of detail
Returns :
sphere: surf.Surface :
A sphere with d**2+2 vertices and 2*d**2 faces. Seen as the planet
Earth, node 0 and 1 correspond to the north and south poles.
The remaining d**2 vertices are in d circles of latitute, each with
d vertices in them.