Fixed improper tick spacing and axis scaling
This requires an API change:
 - AxisItem.setScale(float) has the usual behavior
 - AxisItem.setScale(None) is no longer allowed. Instead use:
 - AxisItem.enableAutoSIPrefix(bool) to enable/disable SI prefix scaling
Also makes the API more intuitive since these features are now accessed and implemented independently.
Updated documentation to match previous API change
Allow AxisItem.setScale(None) to retain API backward compatibility.
minor fix
Merge branch 'axis_fix' into develop
    Fixed improper tick spacing and axis scaling
    This requires an API change:
     - AxisItem.setScale(float) has the usual behavior
     - AxisItem.setScale(None) is deprecated (but still works). Instead use:
     - AxisItem.enableAutoSIPrefix(bool) to enable/disable SI prefix scaling
    Also makes the API more intuitive since these features are now accessed and implemented independently.

fixes #4