Move brittle-fracture 'quantity evaluation' into a dedicated class

This makes the code easier to understand, and also the separate class will be needed in automated tests of the brittle-fracture stuff.

As part of the change, I rename 'Quantity' to 'Jet'. While not everybody may have heard of the word 'jet', it is a well-established term in differential geometry, where is means "the value of a function of some of its derivatives at a given point". That's pretty close to what we use it for here.