clean up `print_sparse` function

[rileyjmurray]

The print_sparse function in RandBLAS 1.1 is very messy. A minimal improvement (should have the same behavior as the current function) is

template <typename SparseSkOp>
void print_sparse(const SparseSkOp &S0) {
    using std::cout;
    constexpr auto tab = '\t';
    cout << "SparseSkOp information\n";
    cout << tab << (S0.dist.major_axis == Axis::Short
        ? "SASO: short-axis-sparse operator"
        : "LASO: long-axis-sparse operator") << '\n';
    cout << tab << "n_rows = " << S0.dist.n_rows << '\n';
    cout << tab << "n_cols = " << S0.dist.n_cols << '\n';

    int64_t nnz = S0.dist.full_nnz;
    auto print_array = [&](const char* label, auto* ptr) {
        cout << tab << label;
        if (ptr != nullptr) {
            cout << "\n" << tab << tab;
            std::copy(ptr, ptr + nnz, std::ostream_iterator<decltype(*ptr)>(cout, ", "));
            // ^ That prints a comma separated sequence to cout.
        } else {
            cout << " is the null pointer.";
        }
        cout << '\n';
    };
    print_array("vector of row indices",    S0.rows);
    print_array("vector of column indices", S0.cols);
    print_array("vector of values",         S0.vals);
}

It would be better if we used RandBLAS' utility functions for array printing, and allowed Matlab / Python formatting strings.