IbdZeroOrder
~~~~~~~~~~~~

Description
^^^^^^^^^^^
Computes IBD cross section in a zero order of perturbation theory, `xsec`
transformation,  and
transforms :math:`E_{\text{e}} \rightarrow E_{\nu}`, `Enu` transformation.

Inputs
^^^^^^

  1. One-dimensional array of :math:`E_\text{e}` for both transformations.

Outputs
^^^^^^^

1. One-dimensional array of :math:`\sigma_{\text{IBD}}`.
2. One-dimensional array of :math:`E_{\nu}`.

Implementation
^^^^^^^^^^^^^^
See the Daya Bay paper on oscillation analysis based on 1230 days of data.