Signature Description Parameters
template<typename T, typename I = unsigned long, typename = typename std::enable_if<std::is_arithmetic<T>::value, T>::type>
struct CorrVisitor;
This functor class calculates the correlation of two given columns.
          explicit CorrVisitor (bool bias = true);
        
See this document and dataframe_tester.cc for examples.
T: Column data type. T must be an arithmetic-enabled type
I: Index type.
    std::cout << "\nTesting Correlation Visitor ..." << std::endl;

    CorrVisitor<double> corr_visitor;
    auto                fut10 = df.visit_async<double, double>("dbl_col", "dbl_col_2", corr_visitor);
    const double        corr = fut10.get().get_result();

    assert(fabs(corr - -0.358381) < 0.000001);