Feature request: pandas.Series.gradient()
#### Problem description
It is common to difference a time series multiple times to stationarize it.
I found surprinsing pandas, which is time-series friendly, does not implement this already and requires to iteratively apply pandas.Series.diff()

#### Expected Begaviour
Same as [np.gradient()](https://docs.scipy.org/doc/numpy/reference/generated/numpy.gradient.html)


