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Computes the outer-product Matrix of the given set of data:
XXt = [ x1 x2 ... xn ] * [ x1 x2 ... xn ]^T.
The outer-product data Matrix is useful in things like computing the
Principal Components Analysis of the dataset. For exapmle, finding the
eigenvectors of the outer-product data Matrix is equivalent to finding
the left singular Vectors ("U" from the SingularValueDecomposition) of
the dataset.
Note that if the input dataset has a size of "N" and each Vector in the
dataset has "M" dimensions, then the return Matrix (XXt) is an (MxM)
Matrix. This method computes the return Matrix without explicitly
forming the data matrix, potentially saving quite a lot of memory.