fix: support scalar/2-elem dim shorthand in tranimate/Animate - #202

Summary

tranimate(T, dim=1.5) — using the scalar-shorthand dim convention
established by plotvol2/plotvol3 elsewhere in this codebase — currently
crashes with:

TypeError: spatialmath.base.animate.Animate() got multiple values for keyword argument 'dim'

Two compounding bugs, both fixed here:

1. tranimate() popped "dims" (plural) out of kwargs before forwarding
   to Animate(dim=..., **kwargs), but every other function in this
   codebase (plotvol2, plotvol3, Animate.__init__ itself) names the
   parameter dim (singular). A caller correctly using dim= per that
   convention never got popped, so it stayed in **kwargs and collided
   with the explicit dim=dim already being passed — hence "multiple
   values for keyword argument 'dim'".
2. Even with the right keyword, Animate.__init__ had its own hand-rolled
   length check that only accepted a 2- or 6-element dim, silently
   dropping the scalar-shorthand convention (A → [-A, A] on every
   axis) that plotvol3 already supports via the shared expand_dims()
   helper — Animate just never called it.

Fix

• tranimate() now accepts dim, with dims kept as a back-compat alias
  for existing callers following the old docstring examples.
• Animate.__init__ now calls expand_dims(dim, nd=3) (the same helper
  plotvol3 uses) instead of its own incomplete validation, so scalar and
  2-element shorthand work consistently everywhere.

Regression context

This isn't a long-standing issue — it's a regression from #131 (Jien Cao,
2024-07-29, fixing #126), which introduced both the dims rename and the
new strict length check while trying to "expose ax and dims arguments
to animate(...), to be consistent with other APIs." Before that commit,
tranimate passed **kwargs straight through to Animate, and scalar
dim worked correctly via Animate's existing plotvol3 delegation.

Test plan

• Verified directly: Animate(dim=1.5), Animate(dim=[0,5]),
  tranimate(R, dim=1.5, movie=True), and tranimate(R, dims=[0,5], movie=True) (back-compat alias) all construct/run correctly
• Full existing test suite: 342 passed, no regressions