NodeMaterial: Rename SwitchNode -> SplitNode

[sunag]

Description

I think that this name would be better for the purpose of the node.

Example

// roughness is in G channel, metalness is in B channel
material.roughnessNode = new SplitNode( mpMapNode, 'g' );
material.metalnessNode = new SplitNode( mpMapNode, 'b' );

What do you think?

/ping @mrdoob @Mugen87

[mrdoob]

Thanks!