Add point_acceleration.

[tkoolen]

Fixes #471. @rdeits, do you also want a

point_acceleration(
    state::MechanismState, v̇::SegmentedVector{JointID}, point::Point3D)

?

[codecov-io]

Codecov Report

Merging #472 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #472      +/-   ##
==========================================
+ Coverage   99.17%   99.17%   +<.01%     
==========================================
  Files          28       28              
  Lines        1580     1582       +2     
==========================================
+ Hits         1567     1569       +2     
  Misses         13       13

Impacted Files	Coverage Δ
src/spatial/spatialmotion.jl	100% <100%> (ø)	⬆️

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[tkoolen]

Merging this now, but if you want that other method, let me know.

[rdeits]

Thanks! I think this method should be sufficient, but I'll let you know