Function partial_eigen_scratch 

pub fn partial_eigen_scratch<T: ComplexField>(
    A: &dyn LinOp<T>,
    n_eigval: usize,
    par: Par,
    params: PartialEigenParams,
) -> StackReq

computes the size and alignment of required workspace for computing the n_eigval eigenvalues (and corresponding eigenvectors) of $A$ with the largest magnitude.