Convenience functions on PlanarJoint for Isometry2d conversions

[mkoval]

#322 added helper functions to FreeJoint, EulerJoint, and BallJoint for setting generalized coordinates from more intuitive datatypes (e.g. Isometry3d). It would be helpful to have a similar helper function on PlanarJoint to convert its generalized coordinates to and from Isometry2ds.

[jslee02]

This is a good idea. I'll add this once GeometricJoint is implemented. GeometricJoint is a more generic version of MultiDofJoint that can handle more configuration spaces including SE(2/3), SO(2/3), and real vector space. GeometricJoint will be able to handle the conversions in general way.

[stale]

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