Signature Description Parameters
template<typename T, typename I = unsigned long, typename = typename std::enable_if<std::is_arithmetic<T>::value, T>::type>
struct SLRegressionVisitor;
This functor class calculates simple linear regression between x and y columns
It produces slop, intercept, and correlation
T: Column data type.
I: Index type.
    std::cout << "\nTesting SLRegression Visitor ..." << std::endl;

    SLRegressionVisitor<double> slr_visitor;

    df.visit<double, double>("dbl_col", "dbl_col_2", slr_visitor);
    assert(slr_visitor.get_count() == 8);
    assert(fabs(slr_visitor.get_slope() - -0.0561415) < 0.00001);
    assert(fabs(slr_visitor.get_intercept() - 0.602674) < 0.00001);
    assert(fabs(slr_visitor.get_corr() - -0.358381) < 0.00001);
    assert(fabs(df.visit<double, double>("dbl_col", "dbl_col_2", corr_visitor).get_result() - -0.358381) < 0.00001);