Allow for getting current position when stepper motor is moving

[alexdewar]

Currently we're using the SP command, which retrieves the set position. However, it won't work if the motor is moving, so we return None in this case. There's also a similar artificial limitation in the DummyStepperMotor device for testing purposes.

It would be useful to know the motor's position even while it's moving, though: if we could record this information in the CSV file then that's potentially extra data and it would be useful to be able to see the position in the GUI too (#583).

According to the manual, there is another command -- IP -- which retrieves the immediate (estimated) position, with this caveat: "This is the calculated trajectory position, which is not always equal to actual
position."

It would be worth playing with this to see if we can just replace SP with IP. Alternatively, we could just use IP only when the motor is moving.

[alexdewar]

I've had a play with this in the lab and it seems that the IP command gives the same result as SP if the motor is stationary, but will also give a sensible-seeming intermediate value if the motor is in motion.

I hereby propose that we drop SP in favour of IP.

[dc2917]

Need to consider what to do for dummy stepper motor. Save current position and return that, or interpolate?

@alexdewar Let's open an issue to poll the intermediate position to update the front-end (stepper motor panel and data file writer)