Book 3: Isotropic: missing method updates

[Walther]

In Book 3, the Isotropic material does not have an updated scatter method and a scattering_pdf method. This makes it the only material that does not get the update treatment in the book, and depending on implementation language and abstractions, can leaveConstantMedium broken / unusable.

[trevordblack]

Can you add comments to #885