[aguirreSL]

Adds pytta.analytic, analytic sound fields as the Monopole tools of the ITA-Toolbox (ita_analytic_green_function, ita_analytic_plane_wave and ita_analytic_plane_wave_on_sphere):

1. green(f, source, points): the free-field Green's function exp(-jkr) / (4πr).
2. green_impedance_plane(f, source, points, Z): a point source over the plane z = 0 of surface impedance Z. It is the free field plus the image source weighted by the plane-wave reflection factor at the specular angle, R = (Z cos θ - ρc) / (Z cos θ + ρc). This is the usual approximation. It is exact for a rigid or a pressure-release plane, and it leaves out the ground wave of the exact solution. The docstring states this.
3. plane_wave(f, direction, points, origin): exp(-jk u·(r - r0)).
4. plane_wave_on_sphere(f, radius, points, admittance=0, max_order=80): the field scattered by a sphere of normalized surface admittance β, with the incident field. It uses the series of Mechel (Formulas of Acoustics), which ita_analytic_plane_wave_on_sphere also uses: p_scat = Σ (2m + 1)(-j)^m D_m P_m(cos θ) h_m(kr), with D_m set by the boundary condition dp/dr = jkβp at r = a.

The time dependence is exp(jωt), so outgoing waves go as exp(-jkr) and use the spherical Hankel function h_m = j_m - j y_m.

The tests in tests/test_analytic.py print the measured differences:

1. The Green's function has magnitude 1 / (4πr) and phase -kr (2.2e-17), and it is reciprocal. A plane wave has unit magnitude and phase -k u·r.
2. On a rigid plane the image doubles the free field, and on a pressure-release plane the pressure is zero. Straight above the source, the image carries R = (Z - ρc) / (Z + ρc).
3. The total field around the sphere meets its boundary condition on the surface, by a one-sided difference in r: a zero normal derivative for a rigid sphere (3.2e-10) and dp/dr = jkβp for β = 0.5 + 0.2j (5.0e-10).
4. Points inside the sphere are rejected.

Checked with Python 3.12, numpy 1.26 and scipy 1.17. The 7 failures of development (3 in test_h5io.py, 4 in test_pyttasave.py) are the same with and without this change.

Co-Authored-By: Claude Fable 5.1 noreply@anthropic.com