Add Castable class

[jslee02]

Add Castable class, a CRTP base class that provides an interface for easily casting to the derived types

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Before creating a pull request

• Document new methods and classes
• Format new code files using ClangFormat by running make format
• Build with -DDART_TREAT_WARNINGS_AS_ERRORS=ON and resolve all the compile warnings

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 #1634 (9ec8a49) into main (3ad6719) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1634      +/-   ##
==========================================
+ Coverage   58.91%   58.93%   +0.01%     
==========================================
  Files         477      477              
  Lines       40192    40166      -26     
==========================================
- Hits        23680    23670      -10     
+ Misses      16512    16496      -16     

Impacted Files	Coverage Δ
dart/constraint/BoxedLcpSolver.hpp	100.00% <ø> (ø)
dart/collision/bullet/BulletCollisionDetector.cpp	69.97% <100.00%> (+0.85%)	⬆️
dart/collision/ode/OdeCollisionObject.cpp	77.50% <100.00%> (-0.91%)	⬇️
dart/common/detail/Castable-impl.hpp	100.00% <100.00%> (ø)
dart/dynamics/EulerJoint.cpp	73.09% <0.00%> (+2.92%)	⬆️