Function qip::iterators::fold_for_op_cols

pub fn fold_for_op_cols<P: Precision, T, F: Fn(T, (u64, Complex<P>)) -> T>(
    nindices: u64, 
    row: u64, 
    op: &PrecisionUnitaryOp<P>, 
    init: T, 
    f: F
) -> T

Fold across rows hitting nonzero columns.