Add hasBodyNode() and hasJoint() to MetaSkeleton

[jslee02]

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Before creating a pull request

• Document new methods and classes

Before merging a pull request

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

[codecov]

Codecov Report

Merging #1000 into release-6.4 will increase coverage by <.01%.
The diff coverage is 61.53%.

@@               Coverage Diff               @@
##           release-6.4    #1000      +/-   ##
===============================================
+ Coverage        56.81%   56.81%   +<.01%     
===============================================
  Files              310      310              
  Lines            24104    24117      +13     
===============================================
+ Hits             13695    13703       +8     
- Misses           10409    10414       +5

Impacted Files	Coverage Δ
dart/dynamics/ReferentialSkeleton.hpp	100% <ø> (ø)	⬆️
dart/dynamics/Skeleton.hpp	83.33% <ø> (ø)	⬆️
dart/dynamics/MetaSkeleton.hpp	100% <ø> (ø)	⬆️
dart/dynamics/ReferentialSkeleton.cpp	39.89% <0%> (-0.36%)	⬇️
dart/dynamics/Skeleton.cpp	66.47% <100%> (+0.15%)	⬆️