Implement a post-scan verification function

[ulrikpedersen]

When a scan is complete it is important to know whether an acceptable scan was performed. A function should be implemented that can do a few basic checks to verify that the mechanism of the scan worked as desired/expected:

• Check that motor was stopped at each acquisition point (for step scans)
• Check that motor position incremented by the requested distance between each step
• Check that the scan output data file exist on disk

Several other checks can be considered as well, of course...