Negative adjusted R-squared values and NaN CIs

[TomiSzucs]

When the adjusted R-squared is negative, then the numerator in the standar error formula is negative as well, as it would involve taking the square root of a negative number. Consider some options:

• Communicate why NaN is displayed to the user in a short format on the output.
• Raise a warning as a negative adjusted R-squared is likely to be the result of some anomaly in the data.