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)^21}.
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.