Mirrors the C# LinearRegression class of the Numerics library: estimates
Y = alpha + beta*X + e, e ~ N(0, sigma), via SVD.
Value
a corehydro_lm list with coefficients, standard_errors, covariance,
residuals, r_squared, adj_r_squared, sigma, df, and n. covariance is the
coefficient covariance matrix, i.e. sqrt(diag(covariance)) equals standard_errors; the
underlying C# LinearRegression.Covariance is the unscaled cross-product term
((X'X)^-1), scaled here by sigma^2 to match standard_errors.