add_totals_row() fails on grouped data_frame

[sfirke]

When called on a grouped data.frame, this produces an error, because of the helper function check_all_cols_after_first_are_numeric() and its select(-1). You can't select out a grouping variable.

mtcars %>%
  group_by(mpg) %>%
  add_totals_row()

Note that I have a branch that needs tests, where add_totals_row works with non-numeric columns and thus eliminates the need for check_all_cols_after_first... - maybe that'll solve this too.

[sfirke]

Yeah, fix this with #57 - add tests and do it on that branch.