Normalize the axis in UnitQuaternion.AngVec - #266

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Thanks very much for the report and the fix, @wpyaoxueros. I've confirmed the bug: on master UnitQuaternion.AngVec(0.5, [1, 2, 3]) returns a quaternion with norm ≈ 1.34, because the axis isn't normalized and the constructor is called with norm=False, check=False. The change and the regression tests look good to me.

Two small things, neither blocking:

1. Exact zero test. if theta == 0: is an exact float comparison, whereas the rest of the library uses a tolerance, e.g. spatialmath.base.iszero(theta) (tolerance in units of eps, as used by unitvec and iszerovec). With the exact test, an angle like 1e-20 with a zero axis raises ValueError, while 0 returns the identity. Using smb.iszero(theta) would make that boundary consistent with the rest of the package.
2. Description. The new ValueError for a nonzero angle with a zero axis is not consistent with the base angle-axis functions: angvec2r(0.5, [0, 0, 0]) (and so SO3.AngleAxis, angvec2tr etc.) currently returns the identity silently, whatever the angle. I think the new AngVec behaviour is the better one, so no change is needed here. I'll open a separate PR to align the base functions with it.

Thanks for confirming the bug and for both suggestions. I've updated AngVec to use smb.iszero(theta) after converting the angle to radians, and clarified the method's note. Added positive/negative near-zero cases in radians and degrees, plus controls ensuring a small angle outside the tolerance still produces its rotation with a valid axis and raises for a zero axis.

The four near-zero subtests fail on the previous PR head and pass with the update. Remote validation: focused tests 12 passed plus 8 subtests; full suite 414 passed, 11 skipped plus 63 subtests. Black 23.10.0 and whitespace checks pass.

You're also right about the description: the base angle-axis functions currently return the identity for a zero axis, including at a nonzero angle. I've corrected the PR text and left those functions unchanged for your separate alignment work.