Add CatVector

[tkoolen]

Will be needed for upcoming changes related to simultaneous simulation of multiple mechanisms (https://github.com/JuliaRobotics/RigidBodyDynamics.jl/issues/572, which might still take a while).

TODO:

• [ ] basic documentation
• [ ] adjust to incoming changes
• [ ] benchmark operations needed for ODE integration

[codecov-io]

Codecov Report

Merging #577 into master will decrease coverage by 0.18%. The diff coverage is 80.3%.

@@           Coverage Diff            @@
##           master   #577      +/-   ##
========================================
- Coverage   89.19%    89%   -0.19%     
========================================
  Files          51     52       +1     
  Lines        3148   3212      +64     
========================================
+ Hits         2808   2859      +51     
- Misses        340    353      +13

Impacted Files	Coverage Δ
src/custom_collections/custom_collections.jl	100% <ø> (ø)	:arrow_up:
src/custom_collections/CatVector.jl	80.3% <80.3%> (ø)
src/custom_collections/SegmentedVector.jl	85.71% <0%> (-0.78%)	:arrow_down:
src/contact.jl	76.62% <0%> (ø)	:arrow_up:

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