Small fixes and features

[janbruedigam]

• Fix Capsule inertia
• Add CombinedShapes shape
• Add function for merging bodies connected by fixed joints

[codecov-commenter]

Codecov Report

Merging #65 (eed5f81) into main (0ff6c2d) will increase coverage by 0.09%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
+ Coverage   87.71%   87.81%   +0.09%     
==========================================
  Files          94       94              
  Lines        5154     5219      +65     
==========================================
+ Hits         4521     4583      +62     
- Misses        633      636       +3     

Impacted Files	Coverage Δ
src/joints/impulses.jl	100.00% <ø> (ø)
src/mechanism/methods.jl	89.83% <77.77%> (-10.17%)	⬇️
src/visuals/visualizer.jl	56.08% <87.50%> (+0.35%)	⬆️
src/bodies/shapes.jl	86.00% <96.66%> (-0.14%)	⬇️
src/gradients/state.jl	89.92% <100.00%> (+0.15%)	⬆️
src/mechanism/set.jl	68.57% <100.00%> (+5.85%)	⬆️
src/mechanism/urdf.jl	92.93% <100.00%> (ø)
src/orientation/rotate.jl	100.00% <100.00%> (+38.46%)	⬆️
... and 1 more

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