Investigate binding and merging DAGs

[malcolmbarrett]

As discussed in #41. Right now, a single dagitty object is associated with a single tidy_dagitty object, but it would be possible to manage multiple DAGs and map through them when using the helper functions.

[ASKurz]

woot!

[malcolmbarrett]

@ASKurz I'm bringing my attention back to this issue, and I'm a little on the fence about how valuable it is.

A few years on, what do you think? Do you consider this to be a missing feature? I haven't had anyone else request it but I do understand your original example linked above

[ASKurz]

Hey @malcolmbarrett, thanks for revisiting the issue. Now we have the patchwork package, the issue might be mute. Make and save the plots for individual DAGs one at a time, and then combine them with patchwork syntax. But I do still think it'd be cool to just use good-old facet_wrap().