Add FissionYieldCutoffHelper for weighting yields by a cutoff energy

This helper performs the following tasks to weight fission yields:
1) Determine a set of "fast" and "thermal" yields
based on a user specified cutoff energy,
2) Tally fission rates above and below the cutoff, and
3) Compute the effective yield by weighting the fast and
thermal yields by the fraction of fast and thermal fissions
for all nuclides in all burnable materials.

The user is allowed to specify the cutoff energy and energies
preferred for the fast and thermal yields. By default, the
cutoff is 112 eV, chosen as it is the logarithmic mean of
0.0253 eV and 500 keV. These two values are the default energies
for thermal and fast yields.

Tests are added to check for failure in constructing this
helper (thermal energy > cutoff, non-real values passed, etc.),
selection of yields given a range of user data, and
a proxy class that emulates a variety of thermal and fast splits
to check the eventual computed yield libraries.

docs/source/pythonapi/deplete.rst

@@ -80,6 +80,7 @@ data, such as number densities and reaction rates for each material.
    ChainFissionHelper
    ConstantFissionYieldHelper
    DirectReactionRateHelper
+   FissionYieldCutoffHelper
    OperatorResult
    ReactionRates
    Results