OAV centring device - add logic for errors as a sanity check

For i04 centring device we want to make sure that the centre is correct with some confidence. fit a guassian to the array at calculated cX and cY and compare mu with the data and check that it is within some range say +- 5 pixels. Also get a value for the residuals of data vs fit and check that this number is not too far from 0. If the residuals is high we know that something has gone wrong.
