Refactor ConstraintSolver: decouple LCP solver from ConstraintSolver

[jslee02]

This PR refactors the current constraint solver, ConstraintSolver, without changing the functionalities. Currently, ConstraintSolver is strongly tied with the (boxed) LCP solver, which leads to that it's not extensible to other non-LCP based methods.

Summary of Changes

• Remove the LCP solver dependency from ConstraintSolver
• Create BoxedLcpSolver, the interface class only for solving boxed LCP problems
• Create Dantzig/PgsBoxedLcpSolver, implementations of BoxedLcpSolver
• Create BoxedLcpConstraintSolver, an implementation of ConstraintSolver that requires a LCP solver. This class is the mostly close class to the previous ConstraintSolver
• Deprecate DantzigConstraintSolver and PgsConstraintSolver in favor of BoxedLcpConstraintSolver + Dantzig/PgsBoxedLcpSolver
• Add a "box stacking" example (will be improved further in the future PRs)

As a note, this is prework for implementation of sequential impulse method of Bullet and hopefully more constraint solvers.

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[codecov]

Codecov Report

Merging #1099 into release-6.7 will decrease coverage by 0.27%.
The diff coverage is 59.58%.

@@               Coverage Diff               @@
##           release-6.7    #1099      +/-   ##
===============================================
- Coverage        56.61%   56.33%   -0.28%     
===============================================
  Files              320      327       +7     
  Lines            24539    24811     +272     
===============================================
+ Hits             13893    13978      +85     
- Misses           10646    10833     +187

Impacted Files	Coverage Δ
dart/math/Helpers.hpp	89.13% <ø> (ø)	⬆️
dart/constraint/ConstrainedGroup.hpp	100% <ø> (ø)	⬆️
dart/constraint/DantzigLCPSolver.cpp	0% <ø> (-59.1%)	⬇️
dart/constraint/DantzigBoxedLcpSolver.hpp	100% <100%> (ø)
dart/constraint/PgsBoxedLcpSolver.hpp	100% <100%> (ø)
dart/constraint/BoxedLcpConstraintSolver.hpp	100% <100%> (ø)
dart/constraint/BoxedLcpSolver.hpp	100% <100%> (ø)
dart/constraint/DantzigBoxedLcpSolver.cpp	22.22% <22.22%> (ø)
dart/constraint/ConstraintSolver.cpp	65.19% <50.87%> (-0.6%)	⬇️
dart/constraint/BoxedLcpConstraintSolver.cpp	56.64% <56.64%> (ø)
... and 15 more

[scpeters]

cross-referencing an issue with kinematic loops in gazebo after this was released in 6.7:

• https://bitbucket.org/osrf/gazebo/issues/2605/kinematic-loops-in-dart-not-working-with