Support numpy >= 2.4 and relax the numpy requirement to <3

[pmrv]

Two changes are needed to work with numpy 2.4, both backward compatible down to numpy 1.x:

1. PyACECalculator/PyACEEnsembleCalculator filled the ASE results dict via np.float64(energy.reshape(-1,)). numpy 2.4 expired the 'conversion of ndim > 0 arrays to scalars' deprecation (present since numpy 1.25), so the scalar constructor now returns a shape-(1,) array instead of collapsing it to a scalar, and atoms.get_potential_energy() leaked a 1-element array of the total energy. Scalar results now go through float(), which behaves identically on every numpy version, and per-atom arrays through astype(np.float64), which also keeps energies_dev/forces_dev arrays for single-atom structures on older numpy.

2. pyace.radial used np.trapz, which numpy 2.0 kept only as a deprecated alias of np.trapezoid and numpy 2.4 removed. The module now picks whichever of the two exists.

A regression assertion in tests/test_PyACECalculator.py guards the scalar energy output. Verified by building the package and running tests/test_PyACECalculator.py against numpy 1.26.4, 2.3.5 and 2.4.6.

Claude-Session: https://claude.ai/code/session_015Ui6Drmop9gZXo1fgn8yVg

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