[ENH] Allow comparisons without groups

[arokem]

For example a model: y ~ s(x, by="t"), where t is also a continuous variable. This would not currently work, because tractr_single_bundle is written for group comparisons (and only binary at that!) and expects something like comp_list = c("0", "1") as input.

[arokem]

For now, it seems that users can call directly to fit_gam setting group.by to NULL.

[arokem]

This might now just work, but we need to add testing that shows this either way.