Add AveragedFissionYieldHelper operator helper

Computes the effective fission yields for nuclides
with multiple sets of yields by
1) Tallying fission rate and energy-weighted fission rate
2) Determining average energy at which fission events occur
3) Interpolating [lin-lin] between adjacent fission yields based
   on this average energy

Uses a second tally with an EnergyFunctionFilter to tally
E * sigma_f * phi.

Added unit tests with the proxy-style class that doesn't use the
C API to generate tallies but tests all other aspects of the
implementation. Tests cover three possible average energies
(below minimum supplied yield energy, above max supplied yield energy,
and between two)

The Operator uses fission_yield_mode="average" to select this helper.
The helper also has a from_operator class method.

docs/source/pythonapi/deplete.rst

@@ -77,6 +77,7 @@ data, such as number densities and reaction rates for each material.
    :template: myclass.rst
 
    AtomNumber
+   AveragedFissionYieldHelper
    ChainFissionHelper
    ConstantFissionYieldHelper
    DirectReactionRateHelper