Implement analytical bunch electric field model for the `HollowDCBeam` bunch shape model
Three cases:
E(r) =
\begin{cases}
0 &, r < R_1 \\
\frac{\rho_0}{2\epsilon_0} \frac{1}{r} \left( r^2 - R_1^2 \right) &, R_1 \leq r \leq R_2 \\
\frac{\rho_0}{2\epsilon_0} \frac{1}{r} \left( R_2^2 - R_1^2 \right) &, R_2 < r
\end{cases}
where
\rho_0 = \frac{I}{\pi \left( R_2^2 - R_1^2 \right) v \gamma}
is the charge density in the bunch frame with I the beam current in the lab frame, v the velocity of the beam and gamma the relativistic gamma factor.