Allow for optional cross sections

[smpark7]

GenericMoltresMaterial and MoltresJsonMaterial currently require all types of neutronics material data (BETA_EFF, CHIT, CHIP, CHID, DIFFCOEF, FISS, INVV, KAPPA, LAMBDA, NSF, NUBAR, REMXS, SP0) regardless of whether they're used in a particular problem.

For example, a steady-state criticality problem without delayed neutrons does not use delayed neutron data (BETA_EFF, CHID, LAMBDA), but the Material objects require users to provide that data. This issue occurs when users wish to use their own neutronics data for such problems instead of parsing a Serpent 2 or SCALE output file with our dedicated python script. The current workaround involves providing said data files filled with meaningless numbers.

This issue can be closed when the Material objects have flags for different types of reactor simulations that determine which neutronics data Moltres requires.