Formula momentum conversion

• Add two functions to Utilities/Util
• Update documentation

Summary

I'm not sure if this is a bug, but I don't understand the formula used to convert P [eV/c] to \beta\gamma [dimensionless] in the function Distribution::converteVToBetaGamma:

(\beta_x\gamma)=\sqrt{(\frac{P_x[{eV/c}]}{m_0c}+1)^2-1}.

If the user gives momentum in eV/c, I would imagine that the conversion should be

\gamma\beta_x = \frac{P_xc}{mc^2}.

If it's not a bug then I think it should be better explained in the manual.