Sync series and dataframe when subsampling

Make sure that both the subsampling series and the dataframe are sliced
when calling statistical_inefficieny(). This ensure that the series and
the dataframe are in sync before the subsampled indices of the series
are applied to the dataframe.

Fixes alchemistry#198

src/alchemlyb/preprocessing/subsampling.py

@@ -295,6 +295,7 @@ def statistical_inefficiency(df, series=None, lower=None, upper=None, step=None,
             raise ValueError("series and data must be sampled at the same times")
 
         series = slicing(series, lower=lower, upper=upper, step=step)
+        df = slicing(df, lower=lower, upper=upper, step=step)
 
         # calculate statistical inefficiency of series (could use fft=True but needs test)
         statinef  = statisticalInefficiency(series, fast=False)