Fix most errors and warnings on Julia nightly

[tkoolen]

Still to do:

• Base.linearindexing deprecation
• Errors due to Dispatch issues with size() on v0.6-dev JuliaArrays/StaticArrays.jl#106

[codecov]

Codecov Report

Merging #176 into master will decrease coverage by -0.35%.
The diff coverage is 96.66%.

@@            Coverage Diff             @@
##           master     #176      +/-   ##
==========================================
- Coverage   92.55%   92.21%   -0.35%     
==========================================
  Files          17       17              
  Lines        1732     1733       +1     
==========================================
- Hits         1603     1598       -5     
- Misses        129      135       +6

Impacted Files	Coverage Δ
src/util.jl	97.89% <ø> (ø)	✅
src/ode_integrators.jl	92.72% <100%> (ø)	✅
src/dynamics_result.jl	100% <100%> (ø)	✅
src/mechanism_state.jl	93.89% <100%> (ø)	✅
src/mechanism.jl	90% <100%> (ø)	✅
src/tree.jl	73.15% <100%> (-4.03%)	❌
src/rigid_body.jl	86.48% <100%> (ø)	✅
src/cache_element.jl	100% <100%> (ø)	✅
src/joint_types.jl	94.35% <100%> (ø)	✅
src/spatial.jl	91.9% <100%> (-1.06%)	❌
... and 5 more

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