Patch for 'normalize' that crashed with empty set

When a function is called on an empty serie, the function 'normalize'
encoutered an error.
Now it raises a custom error that is tracked by the function 'evaluateTokens'
and returns []
adding test function for normalize()
Merge remote-tracking branch 'g76r/f_normalize' into lyft_release_001
bugfix in perSecond():  if the series has null data, diff the existing data.
    Consider a series that looks like this:
        X1,null,X2,null,X3,null,...
    In the current implmenentation, perSecond() will return all nulls
    for this series.  With this bugfix, perSecond will return instead:
        (x2-x1)/step,null,(x3-x2)/step,null,...