Fixes for 0.7

[tkoolen]

Skip IntervalArithmetic and SymPy notebooks for now.

Had to temporarily disable test case involving Hessian of kinetic energy due to excessive compilation time.

Currently some allocations that I still need to debug, but overall benchmarks have improved signficantly.

[codecov-io]

Codecov Report

Merging #451 into master will increase coverage by 2.3%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #451     +/-   ##
=========================================
+ Coverage   96.88%   99.18%   +2.3%     
=========================================
  Files          29       28      -1     
  Lines        2695     1601   -1094     
=========================================
- Hits         2611     1588   -1023     
+ Misses         84       13     -71

Impacted Files	Coverage Δ
src/custom_collections.jl	100% <ø> (+4.65%)	⬆️
src/contact.jl	100% <ø> (+5.26%)	⬆️
src/pdcontrol.jl	100% <ø> (ø)	⬆️
src/joint.jl	100% <ø> (+3.36%)	⬆️
src/dynamics_result.jl	100% <ø> (+10.2%)	⬆️
src/parse_urdf.jl	95.6% <ø> (-0.44%)	⬇️
src/ode_integrators.jl	100% <ø> (+2.65%)	⬆️
src/spatial/spatialmotion.jl	100% <100%> (+1.69%)	⬆️
src/rigid_body.jl	94.59% <100%> (+3.1%)	⬆️
src/joint_types.jl	100% <100%> (+3.35%)	⬆️
... and 36 more

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[tkoolen]

Allocations (except in constraint_jacobian!) are fixed by JuliaArrays/StaticArrays.jl#461. I'll at least wait for StaticArrays 0.8.1 to be tagged before merging this.

[tkoolen]

Allocation fixes for constraint_jacobian! and constraint_bias! will come in a separate PR.