Add residualize_over_grid()

[strengejacke]

Fixes #129

[bwiernik]

Could we instead make this a model_residuals() function that takes a newdata argument, which could be a grid from get_datagrid()?

[strengejacke]

Sure, but do we need newdata? The grid is extracted / taken from the estimate_means() (or related) result.

[DominiqueMakowski]

It can still be useful for power-users to have a way to pass the data no?