Spherical wave expansions: quadratic, gaussians and plane waves.

[haakoek]

Add spherical wave expansion functions and examples

This pull request adds three new analytic spherical-harmonic radial-coefficient functions to potentials.py, along with accompanying example scripts.

New functions

• gaussian_spherical_wave_expansion(r, r0, A, alpha, L_max, M_max)
  Computes $g_{LM}(r) = 4\pi A e^{-\alpha(r^2+R_0^2)} i_L(2\alpha r R_0) Y_{LM}^*(\hat{r}_0)$ using scipy.special.spherical_in.

• plane_wave_spherical_wave_expansion(r, k, L_max, M_max, sign)
  Computes $f_{LM}(r) = 4\pi(i\sigma)^L j_L(kr),Y_{LM}^*(\hat{k})$ using scipy.special.spherical_jn. Supports $e^{\pm i\mathbf{k}\cdot\mathbf{r}}$ via the sign parameter.

• quadratic_potential_spherical_wave_expansion(r, r0, omega, L_max, M_max)
  Exact two-term expansion of $\frac{1}{2}\omega^2|\mathbf{r}-\mathbf{r}_0|^2$; only $L=0$ and $L=1$ are non-zero.

All functions follow the existing L_max/M_max/LM_to_I interface and auto-detect real output.

New example scripts

• examples/gaussian_spherical_wave_expansion.py — single Gaussian and symmetric two-centre superposition (odd $L$ cancellation)
• examples/plane_wave_spherical_wave_expansion.py — $e^{+i\mathbf{k}\cdot\mathbf{r}}$ and $e^{-i\mathbf{k}\cdot\mathbf{r}}$ side by side
• quadratic_potential_spherical_wave_expansion.py — single and symmetric two-centre superposition