FIX: use squared FormFactor in EnergyDependentWidth

[redeboer]

Fix-up to #422, which redefined EnergyDependentWidth as (see v0.15.2)

$$\Gamma_{0}\left(s\right) = \frac{\Gamma_{0} \mathcal{F}_{L}\left(s, m_{a}, m_{b}\right) \rho\left(s\right)}{\mathcal{F}_{L}\left(m_{0}^{2}, m_{a}, m_{b}\right) \rho_{0}\left(m_{0}^{2}\right)}$$

instead of (see v0.15.1)

$$\Gamma_{0}\left(s\right) = \frac{\Gamma_{0} B_{L}^2\left(q^2\left(s\right)\right) \rho\left(s\right)}{B_{L}^2\left(q^2_{0}\left(m_{0}^{2}\right)\right) \rho_{0}\left(m_{0}^{2}\right)}\,.$$

This is not equivalent, because FormFactor is defined as:

$$\mathcal{F}_{L}\left(s, m_{a}, m_{b}\right) = \sqrt{B_{L}^2\left(d^{2} q^2\left(s\right)\right)}\,.$$