Calculate the expanding sample covariance.
If not supplied then will default to self and produce pairwise
If False then only matching columns between self and other will be
used and the output will be a DataFrame.
If True then all pairwise combinations will be calculated and the
output will be a MultiIndexed DataFrame in the case of DataFrame
inputs. In the case of missing elements, only complete pairwise
observations will be used.
Delta Degrees of Freedom. The divisor used in calculations
is N - ddof, where N represents the number of elements.
N - ddof
Keyword arguments to be passed into func.
Return type is determined by the caller.