Add a function to calculate thermal factor (u2) from a dos

[granrothge]

As a diagonostic it would be nice to calculate u^2 in addition to the 2W that is currently calculated.
essentially 2W =u^2Q^2

u^2 has the advantage of being Q independent.

I would like an additional function added to src/multiphonon/forward/phonon.py

I think the function would be

def usqcalc(E,dE,g,beta,M):
g0 = gamma0(E, g, beta, dE)
return g0* J2meV * (h_b / A2m) ** 2.0 / 2.0 / (M * amu)