for linear regression add tests of validity

[aayrapet]

-do normality tests
-do tests on E(epsilon)=0
-do heteroscedasticity tests
-do correction of heteroscedasticity , add robust white estimator
-do autocorrelation tests ( DW, partial correlations etc)
-do correction of autocorrelation
-do endogenous instrumental variables
-do IV tests
-add bootstap on errors for significance testing in case of not normal errors