API: Change default name of user_readback.
API: Change default name of user_readback.

A couple reasons:
- backward compatibility with routines developed when motors only returned
  one value
- the vast majority of the time, people want the user_readback, and if they
  need to check the setpoint of course they can still access it

This change will not be made in other places through ophyd; it's just a
really common preference for motors, here codified as a default for
convenience and, moreover, consistency across the facility.

dchabot/ophyd,dchabot/ophyd