Random Seed as Parameter & Added Res Validity Check

[AyrtonB]

This PR lets the user set a random seed value for both the 2d and 3d perlin noise functions

[pvigier]

Hi,

Do we really want to set the numpy seed inside the functions? It may silently mess the global state of applications that consume the library and provoke strange bugs.

[AyrtonB]

Hi @pvigier,

That's a really good point for the case when people don't want control over the seed.

This could be handled by including a if seed is not None and only running np.random.seed(seed) if that's the case.

[Permafacture]

@AyrtonB I've implemented seed setting the way numpy developers recommend now in #15