Avoid division by 0 in Plot Profile.

This PR fixes the situation in which a ``ZeroDivisionError`` is thrown when the ROI line is parallel to the X axis and hence the ``y1`` and ``y2`` values are equal, returning 0 in the subtraction.

The error returned was:
```python
Traceback (most recent call last):
  File "./script", line 453, in <module>
    fileAnns, message = processImages(conn, scriptParams)
  File "./script", line 382, in processImages
    processLines(conn, scriptParams, image, lines, lineWidth, f)
  File "./script", line 264, in processLines
    lineWidth, theZ, theC, theT)
  File "./script", line 93, in getLineData
    rads = math.atan(float(lineX) / lineY)
ZeroDivisionError: float division
```

To test - check that the values returned from the script make sense.
/cc @will-moore