Remove conversion from parameters

It turns out that from Concert we use devices in a higher-level mode as originally anticipated, i.e. the conversion is no longer necessary because the "real" device with which we communicate in getters and setters in most cases uses some units anyway, like e.g. Tango motors. The device implementer does not need to specify the conversion anymore. An example device:

```python
class FooMotor(LinearMotor):
    def _get_position(self):
        # We can use any compatible unit here
        return float(self._connection.execute("POSITION?")) * q.mm

    def _set_position(self, position):
        # we know that the unit is length
        self._connection.send("POSITION: {}".format(position.to(q.mm).magnitude))
```