Support for SEAs

[tkevinbest]

Note - none of the angle calculation code would support SEAs right now. We might want to consider renaming things. For example joint.output_position is calculated just via motor angle with gear ratio. If there's an SEA in the mix, that might be misleading. I'm not sure the best way to handle this, but we can talk about it. Just wanted to get it our radar.

[senthurayyappan]

Yes! I named it output_position because there is another attribute called the joint_position, which is read from a joint encoder. I think this would be the extent we can go before things get too complicated. We can improve our documentation to convey this better.