Change Markers into Nodes

[mxgrey]

Up until now, Markers have been managed separately from Nodes, because Markers predate the Node concept in DART. This PR changes Markers into a Node type so that it shares all the same machinery as Nodes instead of having its own machinery.

Note that this PR builds off of #691

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This change is

[jslee02]

Reviewed 20 of 43 files at r1, 1 of 1 files at r2, 4 of 4 files at r4.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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dart/dynamics/BodyNode.h, line 614 [r3] (raw file):
This function seems useful. How about having another createMarker that takes the marker properties like for ShapeNode and EndEffector for consistency?

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Comments from Reviewable

[mxgrey]

Review status: 2 of 16 files reviewed at latest revision, 1 unresolved discussion.

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dart/dynamics/BodyNode.h, line 614 [r3] (raw file):
I'll also modify the createEndEffector(const Properties&) implementation to no longer be templated. It used to be a templated function in order to work around the fact that EndEffector.h couldn't be visible to BodyNode.h due to a circular dependency. That circular dependency is gone now, so we no longer need the template there.

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Comments from Reviewable

[jslee02]

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Reviewed 14 of 19 files at r5, 4 of 4 files at r6.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable