A configurable version of the vanilla GenLayerRiverMix. Uses a biome map and a river 
map layer to set configurable river biomes for biomes.



Constructor:
    Arguments:
        long, GenLayer, GenLayer
    Argument Usage:
        base layer seed, biome map parent layer, river map layer
    Notes:
        Constructs the layer, setting the base layer seed and parent layers.

setRiverBiome:
    Arguments:
        Non_Null_Biome_ID, Non_Null_Biome_ID
    Usage:
        biome ID, river biome ID
    Notes:
        Sets the river biome for the provided biomeID.