Represents a 4-tensor which is expressible as
kron(out_scatter, in_scatter), where out_scatter
and in_scatter are both positive semi-definite.
This is typically used to express the precision
or covariance of a crate::normal_inverse_wishart::NormalInverseWishart model.
See also crate::func_schmear::FuncSchmear for an example of usage.