Add merge! for animation types, insert! for AnimationTrack.

[tkoolen]

On top of #112.

Needs tests, but seems to be working fine for me. Just wanted to open this for review and so that I don't forget.

[codecov-io]

Codecov Report

Merging #115 into master will decrease coverage by 0.54%.
The diff coverage is 69.23%.

@@            Coverage Diff            @@
##           master    #115      +/-   ##
=========================================
- Coverage   76.04%   75.5%   -0.55%     
=========================================
  Files          14      15       +1     
  Lines         405     449      +44     
=========================================
+ Hits          308     339      +31     
- Misses         97     110      +13

Impacted Files	Coverage Δ
src/lowering.jl	83.09% <ø> (ø)	⬆️
src/MeshCat.jl	30.43% <ø> (ø)	⬆️
src/atframe.jl	83.87% <100%> (+6.72%)	⬆️
src/util.jl	100% <100%> (ø)
src/animations.jl	57.14% <42.85%> (-19.05%)	⬇️

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[rdeits]

This seems totally reasonable. Let's do it!

[tkoolen]

I was just thinking: there could potentially be two animation tracks with the same name but different jstypes. That would currently error here. Again, not with the current atframe implementation,

MeshCat.jl/src/atframe.jl

Lines 12 to 14 in 160933c

	track = get!(clip.tracks, prop) do
	AnimationTrack(prop, jstype, Int[], wider_js_type(typeof(value))[])
	end

but still.

[rdeits]

Thanks!

[tkoolen]

Actually, there's still an issue with merging AnimationTracks. I'm working on a test + fix. I'll open another PR.

[rdeits]

Ok, no problem

[tkoolen]

#124