add_group for internal use, enw_assign_group for external use?

[seabbs]

focus the pure technical capabilities in add_group (such as executing on a by clause), but have it assume (because it's internal function) that it's not handed garbage. the enw_assign_group handles all the crap users might throw at it.

Originally posted by @pearsonca in #208 (comment)

[seabbs]

I think we always want to go via assign group (at least for now) but agree we could have assign group be the exposed to the user version and add_group be the less safe internal use version (though for people inspecting the code this may lead them astray (if for example they want to role their own version of the preprocessing function for some reason).

[seabbs]

Done in #239