Request for a force_finite flag for score function

[thobanster]

The r2_score method in sklearn has a force_finite flag which defaults to True in order to avoid infinite and NaN values when the TSS happens to be 0. The analogous quantity when computing D^2 is the null deviance, which can also sometimes be 0. It would be great if, in glum, there was also a force_finite flag that can gracefully handle the case where the null deviance happens to be 0. Right now, I get a ZeroDivisionError in glum 2.1.2 running in Python 3.6.

[lbittarello]

Very late to the game, but: Why do we use a pseudo-R² as the score at all? Couldn't we just report the negative or inverse deviance? For tuning purposes, it shouldn't change the ranking of parametrisations. @MarcAntoineSchmidtQC @jtilly