[aguirreSL]

Adds pytta.fourpole, the four-pole (two-port) matrices of lumped mechanical and acoustic elements that itaFourpole and ita_make_fourpole of the ITA-Toolbox provide. The transmission matrix A relates force and velocity at the input to those at the output, [F1, v1] = A [F2, v2], and the matrices are arrays of shape (nFreqs, 2, 2):

1. element(kind, value, freqs): the matrix of a mass, a spring (by its compliance), a dashpot, an impedance in series or to ground, a transformer or a gyrator, with the conventions of ita_make_fourpole.
2. chain(*matrices): elements connected one after the other.
3. input_impedance(A, load=None): F1 / v1 with a load at the output, blocked by default.
4. a_to_z(A) and z_to_a(Z): the impedance matrix with both velocities entering the element, as itaFourpole.A2Z and Z2A.

The ITA code was read for its semantics only and nothing was copied.

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

1. A mass on a grounded spring has the input impedance jωm + 1 / (jωn) (2.0e-15), and with a free output only the mass is left (1.1e-16).
2. A chain of a mass, a dashpot, a spring and a shunt impedance has Z12 = Z21 (1.7e-14), and A comes back from Z (1.9e-16).
3. A gyrator twice is the identity, and a transformer of ratio t shows a load Z as t² Z.

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