Fix slerp at coincident/antipodal endpoints, and SO2/SE2.interp1()

[gaoflow]

Two crashes in the interpolation methods, found sweeping the whole slerp surface (angle between the endpoints from 0 to pi across both singular ends, s over [0,1], shortest either way).

slerp singularities

The slerp weights sin((1-s)t)/sin(t) and sin(s.t)/sin(t) are singular wherever sin(t) vanishes: at t = 0 (coincident endpoints) and at t = pi (antipodal endpoints, which are the same rotation under the double cover). qslerp() guards only t = 0; UnitQuaternion.interp() and .interp1() re-derive the weights inline and guard neither.

q = UnitQuaternion.Rx(0.3)
q.interp(q, 0.5)                                 # ZeroDivisionError
UnitQuaternion().interp1(0.5)                    # ZeroDivisionError
UnitQuaternion.Rx(pi).interp(UnitQuaternion.Rx(-pi), 0.5, shortest=True)   # ZeroDivisionError
UnitQuaternion.Rx(pi).interp(UnitQuaternion.Rx(-pi), 5)                    # TypeError
qslerp(q.vec, -q.vec, 0.5)                       # [0 0 0 0], not a unit quaternion

acos(dotprod) loses the small angle to rounding at both ends, so sin(acos(dotprod)) is a poor denominator. This takes sin(t) directly as the length of the component of q1 orthogonal to q0, which keeps full relative precision, and gets t from atan2. The only degenerate case left is sin(t) == 0, where the endpoints are the same rotation and so is every interpolate. The two UnitQuaternion methods now call qslerp(), which their own :seealso: already pointed at, so the formula lives in one place.

Checked against a 50-digit q0 exp(s log(q0^-1 q1)) reference — Lie-group form, so it shares no algebra with the sin-weight formula — over the full range of t and s:

• endpoints within 1e-6 of antipodal: worst error 4.4e-5 -> 2.7e-10 rad
• largest deviation from unit norm anywhere: 8.2e6 -> 2.8e-4 (just short of antipodal, qslerp was returning quaternions with a norm in the millions)
• ordinary angles move by at most 1 ulp, and the patched surface agrees with scipy.spatial.transform.Slerp to 1.4e-15 rad over 10000 random pairs

Exactly antipodal with shortest=False has no unique great circle, so there is no correct answer there; it now returns a unit quaternion for the rotation both endpoints share instead of a norm-1e6 vector or [0 0 0 0].

SO2/SE2.interp1()

The fix for #33 dropped the start local but replaced its uses only in the N == 3 branch, so SE2(1, 2, 0.3).interp1(0.5) has raised NameError: name 'start' is not defined ever since — #33 did report it for both SE2 and SE3. interp1() had no test coverage in either dimension.

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4 new test cases, all red on master. The existing test_slerp and test_interp values were already correct and pass unchanged. Full suite green, black 23.10.0 clean.