Broadcast scalar integration steps over twist sequences

[antonsoo]

Twist3.exp documents applying one integration step to every twist in a sequence, but currently raises ValueError: length mismatch. The SE3 conversion also rejects that case. The corresponding Twist2 methods fail too, including matched per-twist steps in SE2.

Handle the shared scalar step in all four methods, and match individual steps in Twist2.SE2. Check degree-mode warnings per twist so that path works for sequences as well. Singleton expansion and mismatched-length rejection are retained.

from spatialmath import Twist3

motion = Twist3([Twist3.Rx(0.2), Twist3.Ry(-0.3)])
print(len(motion.exp(0.5)))  # 2; currently raises ValueError
print(len(motion.SE3()))     # 2; currently raises ValueError

Validation: 20 focused cases compare the results to independent SciPy matrix exponentials (17 fail before). A 717-row camera-reconstruction fixture exercises 96 motion increments in 2D and 3D: all 21 formerly aborting sequence workflows complete, and transformed points agree with the independent reference within 4e-14. Ordinary single-twist output hashes are unchanged. Full suite: 367 passed, 3 existing skips; project pre-commit hooks pass. Linux/Python 3.12.