Review PSISimplePositionerBase integration

[cappel89]

• Review the PSISimplePositionerBase class and improve readability of cross-dependencies for optional signals. Ideally, a Readback PV should be sufficient as minimum requirement.
• Define fine-grained classes in devices.simple_positioner with the following signal combinations: ('readback'); ('readback', 'setpoint'), ('readback', 'motor_done_move'), ('readback', 'setpoint', 'motor_done_move'), ('readback', 'setpoint', 'motor_done_move', 'velocity' )