angvec2r: reject a zero axis for a non-zero angle

[petercorke]

Follow-up to #266, which makes UnitQuaternion.AngVec raise ValueError for a non-zero angle about a zero axis. As noted there, the base functions did not behave consistently with that.

Problem

angvec2r tested the axis norm and returned the identity if it was ~0, whatever the angle:

>>> angvec2r(0.5, [0, 0, 0])   # silently the identity

angvec2tr, SO3.AngleAxis/AngVec and SE3.AngleAxis/AngVec inherit this. The docstring already says that tol is for the "zero-rotation case" and that V must have finite length when the angle is non-zero, but the code did not do that.

Change

Test the angle for zero instead:

• angle zero (within tol eps, via iszero): return the identity, axis ignored
• otherwise: normalize the axis with unitvec, which raises ValueError for a zero-length axis

Docstring notes for angvec2r and angvec2tr updated (the old θ ~= 0 wording was MATLAB "not equal" and read ambiguously).

Behaviour change

A non-zero angle about a zero-length axis (including a tiny axis below 20 eps) now raises ValueError rather than returning the identity. Non-zero axes of any length are unaffected.

Downstream

I audited the sibling toolboxes. The only affected caller is a line-of-sight helper in RTB that applies AngleAxis to cross(a, b), which is zero for (anti)parallel vectors. It is already wrong for antiparallel vectors today, and SO3.RotatedVector (in the released 1.1.18) is the correct replacement. Tracked in petercorke/robotics-toolbox-python#843, to land before this is released.

Tests

• test_angvec2r_nonunit_axis, test_angvec2r_zero_axis (rad and deg), additions to test_angvec2tr, and test_AngleAxis_zero_axis for SO3/SE3
• 3 of the new tests fail on current master and pass with the change; full suite 413 passed, 3 skipped
• Not run through Black: the project pins 23.10.0 and my environment has a newer version, so I formatted by hand and left it to pre-commit/CI

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