Spontaneous fission decreases total decay rate in burnup

[eepeterson]

Bug Description

Since depletion chains store the target for the spontaneous fission decay mode as the isotope itself (shown below) these end up being added back along the diagonal as a "source" when constructing the burnup matrix which incorrectly decreases the destruction rate that would normally be there. It's a very small effect typically, but ultimately should be corrected.

  <nuclide name="Cf252" half_life="83468070.0" decay_modes="2" decay_energy="6032280.0" reactions="5">
    <decay type="alpha" target="Cm248" branching_ratio="0.96908"/>
    <decay type="sf" target="Cf252" branching_ratio="0.03092"/>

Steps to Reproduce

Environment