Account for coat IOR in base roughening

[portsmouth]

As discussed on Slack, the coat roughening formula we suggest in the spec:

incorrectly ignores the IOR of the coat. For example if the coat IOR is 1, the coat is effectively not present so there should be no roughening effect.

Generally, I think we need to do some simulations to check the actual behavior, and come up with a good fitting formula, as the theory seems a bit hand wavy at present.. At least it should be understood that these formulas are just suggested approximations, but it's not good if they're totally wrong in certain regimes.