Most of the surface optics do not respond to SurfaceElemental in calcBsdf()

[TzuChieh]

Need to do the following:

• make them respond
• check for missed ones in other methods
• properly test this (integration test maybe?)

[TzuChieh]

They do. It was a false positive alert. Since input should not provide invalid elemental ID in the first place (>= numElementals).

Now the steps are

• assert ID range

• assert this

• document this

[TzuChieh]

• [x] Check if this is still an issue
• [x] Document the use of elemental