Extended Physics Informed Neural Networks (XPINNs) code to solve the 3D Poisson-Boltzmann equation.

$$\nabla^2 \phi_1 = -\frac{1}{\epsilon_1}\sum_k q_k\delta(x_k) \;;\\ x \in \Omega_1 $$

$$\nabla^2 \phi_2 = \kappa^2\phi_2 \;;\\ x \in \Omega_2 $$