Add support for mimic joints

[costashatz]

This PR adds support for mimic joints (see #733). It does so by creating a new actuator type (MIMIC) and inserting one constraint for each mimic joint. In short:

• Mimic joints are treated as velocity-controlled dynamic joints; similar to servo motors
• A mimic joint will try to follow the (multiplied and offset-ed) position of a "master" joint
• The JointProperties struct now holds three additional variables: mMimicJoint (pointer to "master" joint), mMultiplier and mOffset (these are only used in mimic joints and ignored for all other types of joints)
• DartLoader (i.e., URDF loader) now correctly loads URDF files with mimic joints. Is there a similar functionality/tag in skel files (I know that in sdf there's no such thing)?
• Cloning a skeleton, adjusts the mimic joint pointers

This is working for me nicely so far..

Let me know what you think and then I can proceed in cleaning the code (add a few more warnings and check it a bit more). Do you think it will be good to add a unit-test? I also added 2 unit tests (one for DartLoader and one for the mimic constraint)..

[mxgrey]

Suggested change

	double _offset = 0.0);
	double _mimicMultiplier = 1.0,
	double _mimicOffset = 0.0);

nitpick: To make it clear that these are properties specifically related to the mimic joint actuator type, it would be good to decorate the variable names with mimic.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-6.7@3147c7b). Click here to learn what that means.
The diff coverage is 77.31%.

@@              Coverage Diff               @@
##             release-6.7    #1178   +/-   ##
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  Coverage               ?   56.51%           
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  Files                  ?      345           
  Lines                  ?    25654           
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Impacted Files	Coverage Δ
dart/utils/urdf/DartLoader.hpp	100% <ø> (ø)
dart/dynamics/detail/JointAspect.hpp	66.66% <ø> (ø)
dart/dynamics/Joint.hpp	66.66% <ø> (ø)
dart/dynamics/Joint.cpp	56.81% <100%> (ø)
dart/constraint/ConstraintSolver.cpp	66.23% <100%> (ø)
dart/dynamics/Skeleton.cpp	66.13% <30%> (ø)
dart/dynamics/detail/GenericJoint.hpp	73.71% <35.71%> (ø)
dart/constraint/MimicMotorConstraint.cpp	79.16% <79.16%> (ø)
dart/utils/urdf/DartLoader.cpp	76.7% <90.32%> (ø)

[jslee02]

Thanks for adding this feature to DART!

I have made some minor changes in my branch. Maybe this is not the best way of reviewing, but could you take a look at the following commits and merge my branch into your branch if they look good?:

• 3ce9cce
• c3d5312
• 9cea13e
• 4803dee
• 2fb09fa
• cfd3b2e

Also, it'd be great to update the changelog for this PR.

[costashatz]

@jslee02 your changes seem great! Integrated them and I updated the changelog..

[jslee02]

Looks great to me! @mxgrey Please let me know if you plan to review this PR further. 😄

[costashatz]

Is there any reason that you are not merging this one? Thanks!

[jslee02]

I was waiting for a second review. Let me merge this now.