c=operatorappr(Op,K);
Opout=operatorappr(Opin,T) computes the an operator Opout of the same type as Opin that best approximates the matrix T in the Frobenious norm of the matrix (the Hilbert-Schmidt norm of the operator).
For some operator classes, the approximation is always exact, so that operator(Opout,f) computes the exact same result as T'*f.