Seting default color for NaN in addScalarQuantity

[berendbaas]

While debugging scalar quantities, I find myself often overlooking NaN values, because as it stands, they (seem to) get mapped to zero. It'd be nice if we could specify the color that NaN values should map to, so they're easier to distinguish. perhaps as an extra optional<glm::vec3> argument (if empty, default to current behaviour) argument would help.