added decay_neutron_energy function to match existing decay_photon_energy

[shimwell]

Description

It would be useful to have a openmc.data.decay_neutron_energy to match the already existing openmc.data.decay_photon_energy

For my use case I've been doing a few simulations with activated water and one of the products is N17 which is a neutron emitter. I'm keen to know the emission details of the neutrons

I notice there are other nuclides of (Cm, Cf, Bk, Am, Sm, Th, U, Pu) that also decay via neutron emission.

This PR makes use of the existing decay_photon_energy code and a bit more generic so that is can get neutron or photon sources

Fixes # (issue)

Checklist

• I have performed a self-review of my own code
• I have followed the style guidelines for Python source files (if applicable)

[Ebiwonjumi]

Thanks @shimwell, this would be useful to determine neutron dose in spent fuel, just as the decay_photon_energy is in shutdown dose of fusion systems.