pintk print_info() is buggy

[paulray]

The space bar command in pintk calls the print_info() function that prints out info about the selected TOAs.
This has some bugs that show up when it calls print_chi2(). First, that function references self.selected_resids. This does not exist. It seems to have been replaced with self.selected_prefit_resids and self.selected_postfit_resids, so that needs to be fixed.
There needs to be logic in there to pick the postfit resids if a fit has been run or otherwise use the prefit resids.
Also, if you select only a few points, the chi2 and DOF calculations get messed up (e.g. printing a negative number of degrees of freedom). This should be fixed also, to compute and print the chi2 only if there are more selected TOAs than fit parameters.