exercise4:
  Find best two-part linear model for Mexico population data.
  Data from mexico_population_data.txt involves n = 13 items with dimension d = 2
  Data statistics:
    Min      =  [1.86500e+03 8.25908e+06]
    Max      =  [2.01800000e+03 1.23982528e+08]
    Range    =  [1.53000000e+02 1.15723448e+08]
    Mean     =  [1.95723077e+03 5.12340641e+07]
    Variance =  [1.78540828e+03 1.54580052e+15]
  Graphics saved as "exercise4_mexico_data.png"

  MSE for a single linear model is  269869933103246.97
  Graphics saved as "exercise4_mexico_fitted.png"
  MSE1+MSE2 for a split linear model is  16507130767987.879
  Graphics saved as "exercise4_mexico_fitted2.png"