Get max velo for moving to start of rotation

[DominicOram]

We currently do yield from bps.abs_set(axis.velocity, 120, wait=True) to go to the start of the rotation scan, we should be able to just get the .VMAX of the motor and use that. This has the advantage of being slightly more readable and will update if the motor ever becomes faster/slower.

Note that EpicsMotor doesn't have VMAX at the moment so we will need to add it in