LinearRegression should be a shorcut to polynomial_regression(degree = 1)

With #7 there is no sense of mantaining both linear_regression and polynomial_regression, as linear_regression is a particular case of polynomial_regression with degree = 1

All the code of linear_regression should be deleted, and because Linear Regression is widely used, create a shorcut to polynomial_regression(degree = 1) with the name of linear_regression (maybe a typedef is enough??)