Integrate fuel based material definitions.

[TheBEllis]

FISPACT-II allows users to set their material compositions using setMass, or setFuel. Using setMass, the user provides the total mass, and the mass fraction of each element. FISPACT uses the isotopes abundances from its nuclear data to determine the specific isotope composition of the elements.

Using setFuel, users provide specific isotopes and the number of atoms for each isotope.

Currently only setMass is supported in Fizzy, we need to implement a setFuel equivalent.