Implement cellular noise

[MarcinKonowalczyk]

glm/gtc/noise has implementations of Stefan Gustafson's Perlin and Simplex noise, but not the cellular noise. Would you be open for me to add it?

In line with the original implementation I'd implement:

• vec<2, T, Q> cellular(vec<2, T, Q> const& Position)
• vec<2, T, Q> cellular_fast(vec<2, T, Q> const& Position)
• vec<2, T, Q> cellular(vec<3, T, Q> const& Position)
• vec<2, T, Q> cellular_fast(vec<3, T, Q> const& Position)

[MarcinKonowalczyk]

Alternative 1: instead of cellular and cellular_fast, make it one function with default value of enum .normal or .fast.
Alternative 2: just like alternative 1 but the addition parameter is on the template level.