Computes the centered moving average of size span on the data.
| Parameters: | data : array-like 
 span : int 
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| Returns: | A (subclass of) MaskedArray. : Noting ``k=span//2``, the ``k`` first and ``k`` last data are always masked. : If ``data`` has a missing value at position ``i``, then the result has : missing values in the interval ``[i-k:i+k+1]``. :  |