Add planner_dart component for DART planners

[gilwoolee]

This PR creates planner_dart component and builds all DART planners as planner_dart component.

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Before creating a pull request

• [x] Document new methods and classes
• [x] Format code with make format

Before merging a pull request

• [x] Set version target by selecting a milestone on the right side
• [x] Summarize this change in CHANGELOG.md
• [x] Add unit test(s) for this change

[sniyaz]

Hey @brianhou, can you take another look? Reviving this now, I've made some fixes so Travis should be satisfied.

[sniyaz]

@aditya-vk You have a good question about when to split off a new component. I think the motivation here is a combination of:

1. The amount of code in this component has grown large enough that splitting it off makes sense.
2. The dependency here on OMPL means that none of our planners would build if OMPL was not installed correctly (assuming we were using a single component for all planners like we have now).

If these motivations sound good, I'm happy to create an issue to track this and update our AIKIDO guidelines.

[codecov[bot]]

Codecov Report

Merging #567 (a3654af) into master (62dfb15) will increase coverage by 3.09%. The diff coverage is 76.71%.

@@            Coverage Diff             @@
##           master     #567      +/-   ##
==========================================
+ Coverage   75.59%   78.69%   +3.09%     
==========================================
  Files         243      170      -73     
  Lines        5856     6406     +550     
==========================================
+ Hits         4427     5041     +614     
+ Misses       1429     1365      -64     

Impacted Files	Coverage Δ
include/aikido/common/BSpline.hpp	100.00% <ø> (ø)
include/aikido/common/RNG.hpp	100.00% <ø> (ø)
include/aikido/common/Spline.hpp	100.00% <ø> (ø)
...clude/aikido/common/detail/ExecutorThread-impl.hpp	100.00% <ø> (ø)
include/aikido/common/detail/Spline-impl.hpp	96.96% <ø> (+0.22%)	:arrow_up:
...lude/aikido/common/detail/metaprogramming-impl.hpp	80.00% <ø> (ø)
include/aikido/common/detail/pair-impl.hpp	100.00% <ø> (ø)
include/aikido/constraint/Differentiable.hpp	100.00% <ø> (ø)
...clude/aikido/constraint/DifferentiableSubspace.hpp	100.00% <ø> (ø)
include/aikido/constraint/Projectable.hpp	100.00% <ø> (ø)
... and 244 more