add hasconstant indicator for R-squared and df calculations

[joep]

see also Bug #440151: fvalue and mse_model are -inf if only 1 regressor

The R-squared and df calculations assume that there is a constant among the regressors.

todo: add a hasconstant indicator and adjust calculations for df and R-squared

Notes: pandas did recently a similar change

R-squared is not really well defined without constant and there are several competing definitions, but for the simple case of regression without constant we should use the total sum of squares instead of mean corrected ???

I would keep the R-squared with constant for transformed regressors that loses the constant for the regression, (example of heteroscedasticity).