Fix TypeError from dartpy.dynamics.Node.getBodyNodePtr()

[jslee02]

dartpy.dynamics.Node.getBodyNodePtr() causes TypeError as

>>> b.getBodyNodePtr()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Unable to convert function return value to a Python type! The signature was
	(self: dartpy.dynamics.Node) -> dart::dynamics::TemplateBodyNodePtr<dart::dynamics::BodyNode>

This was because dart::dynamics::TemplateBodyNodePtr<dart::dynamics::BodyNode> is declared after the Node binding. This PR fixes it by declaring the holder type at the top of dynamics submodule.

This PR needs to be merged before #1457.

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[codecov]

Codecov Report

Merging #1463 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1463      +/-   ##
==========================================
+ Coverage   58.23%   58.24%   +0.01%     
==========================================
  Files         412      412              
  Lines       29928    29928              
==========================================
+ Hits        17429    17432       +3     
+ Misses      12499    12496       -3     

Impacted Files	Coverage Δ
dart/dynamics/Skeleton.cpp	66.33% <0.00%> (+0.16%)	⬆️