Add DOM classes for *_state elements

[scpeters]

Desired behavior

The syntax for specifying model, link, and joint states was updated in the SDFormat specification in #1461 (inspired by the prototype in gazebosim/gz-sim#2359). The next step is creating DOM classes for the following SDFormat elements to simplify their use in the C++ and python API:

• Create JointAxisState class for //joint_state/axis_state and //joint_state/axis2_state
• Create JointState class for //joint_state
• Create LinkState class for //link_state
• Create ModelState class for //model_state

Alternatives considered

Implementation suggestion

Follow the patterns used by our existing DOM classes.

• sdf::JointAxis
  • JointAxis.hh
  • JointAxis.cc
  • pyJointAxis.hh
  • pyJointAxis.cc

Additional context