Add pybind wrappers for BodyNode's getChildBodyNode() & getChildJoint()

[chhinze]

• Add pybind11 wrapper for getChildBodyNode(szd::size_t index) -> BodyNode*
• Add pybind11 wrapper for getChildJoint(szd::size_t index) -> Joint*
• Add unit test, verifying that the new wrappers work

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Before creating a pull request

• Document new methods and classes
• Format new code files using clang-format

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 #1387 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1387   +/-   ##
=======================================
  Coverage   57.27%   57.27%           
=======================================
  Files         366      366           
  Lines       27432    27432           
=======================================
  Hits        15711    15711           
  Misses      11721    11721

[jslee02]

Looks good to me. Thanks for your contribution!

[chhinze]

Thank you for merging 👍