Nuclear decay: correct beta energy spectrum

[DavidWalz]

The energy distribution of electrons from beta decay is given by (Basdevant, eq 4.92)
p(E_e) ~ F(Z',E_e) E_e \sqrt{E_e^2 - m_e^2 c^4} (\Delta m c^2 - E_e)^2

Currently beta energies are generated according to the free neutron decay (Delta m = m_n - m_p, F=1) and then approximatively scaled to the given nuclear decay.
Implementing the correct treatment would be straightforward however.

[DavidWalz]

Closed with commit 9e01532