Change BoxedLcpConstraintSolver to use Eigen instead of raw pointer

[jslee02]

Summary of Changes

• Use Eigen objects instead of raw point in BoxedLcpConstraintSolver
• Assume ContactConstraint handles only one contact
• Make LCP related terms as member variable to avoid frequent memory allocation
• Format code of BoxedLcpConstraintSolver and ContactConstraint

Depends on #1099

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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

[codecov]

Codecov Report

Merging #1101 into release-6.7 will decrease coverage by 0.02%.
The diff coverage is 66.43%.

@@               Coverage Diff               @@
##           release-6.7    #1101      +/-   ##
===============================================
- Coverage        56.33%   56.31%   -0.03%     
===============================================
  Files              327      328       +1     
  Lines            24811    24793      -18     
===============================================
- Hits             13978    13962      -16     
+ Misses           10833    10831       -2

Impacted Files	Coverage Δ
dart/constraint/BoxedLcpConstraintSolver.hpp	100% <ø> (ø)	⬆️
...art/dynamics/detail/TranslationalJoint2DAspect.hpp	60% <ø> (ø)	⬆️
dart/common/detail/LockableReference-impl.hpp	36.36% <0%> (+1.06%)	⬆️
dart/constraint/ContactConstraint.hpp	100% <100%> (ø)
dart/constraint/ContactConstraint.cpp	61.53% <64.43%> (+0.12%)	⬆️
dart/constraint/BoxedLcpConstraintSolver.cpp	51.82% <77.77%> (-4.82%)	⬇️