added reset_seed

docs/scarpet/language/SystemFunctions.md

@@ -239,6 +239,11 @@ map(range(10), bool(rand(2))) => [false, false, true, false, false, false, true,
 map(range(10), str('%.1f',rand(_))) => [0.0, 0.4, 0.6, 1.9, 2.8, 3.8, 5.3, 2.2, 1.6, 5.6]
 </pre>
 
+## `reset_seed(seed)`
+
+Resets the sequence of the randomizer used by `rand` for this seed to its initial state. Returns a boolean value
+indicating if the given seed has been used or not.
+
 ### `perlin(x), perlin(x, y), perlin(x, y, z), perlin(x, y, z, seed)`
 
 returns a noise value from `0.0` to `1.0` (roughly) for 1, 2 or 3 dimensional coordinate. The default seed it samples