Accept plane names for the axis argument of Probe.rotate

[adityasingh2400]

The docstring of Probe.rotate documents axis as one of "xy", "yz", or "xz", but passing any of those strings raises ValueError: could not convert string to float: 'xy'. Only a 3-element vector actually works today, so the documented form of the argument has never been usable.

The cause is that rotate forwards axis straight to _rotation_matrix_3d, which immediately does axis = axis / np.linalg.norm(axis). np.linalg.norm calls astype(float) on its input, and a string cannot be cast, so the call dies before the rotation matrix is ever built.

This adds a small _axis_to_vector helper that normalises the argument before it reaches _rotation_matrix_3d. A plane name maps to the unit vector normal to that plane, so "xy" becomes [0, 0, 1], "yz" becomes [1, 0, 0], and "xz" becomes [0, 1, 0], which is the mapping Alessio suggested in the issue thread. Vectors keep working exactly as before, and anything else now raises a ValueError that names the accepted values instead of surfacing a numpy casting error. The type hint and docstring were updated to say that both forms are accepted.

Tested with new cases in tests/test_probe.py. They rotate the same 3D probe twice, once by plane name and once by the equivalent vector, and assert that contact positions and contact plane axes match, plus a check that rotating in the xy plane leaves z untouched and that bad axis values raise. All four new checks fail on main with the numpy casting error and pass with this change. The existing suite still passes, 193 passed and 1 skipped.

Fixes #273