FEAT: implement FormFactor expression class

[redeboer]

Implemented a new FormFactor class to the new ampform.dynamics.form_factor (#418) module. This is an expression class for the formulate_form_factor() function:

$$\mathcal{F}_{L}\left(s, m_{1}, m_{2}\right) = \sqrt{B_{L}^2\left(d^{2} q^2\left(s\right)\right)}$$

The form factor is also used in the definition of EnergyDependentWidth and relativistic_breit_wigner_with_ff(), e.g.:

$$\begin{array}{rcl} \text{Breit-Wigner with FF} & = & \frac{\Gamma_{0} m_{0} \mathcal{F}_{L}\left(s, m_{1}, m_{2}\right)}{m_{0}^{2} - i m_{0} \Gamma_{0}\left(s\right) - s} \\\ \Gamma_{0}\left(s\right) &=& \frac{\Gamma_{0} \mathcal{F}_{L}\left(s, m_{1}, m_{2}\right) \rho^\mathrm{CM}\left(s\right)}{\mathcal{F}_{L}\left(m_{0}^{2}, m_{1}, m_{2}\right) \rho^\mathrm{CM}_{0}\left(m_{0}^{2}\right)} \\\ \end{array}$$

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