Modal element group should be set up unconditionally

> I agree, it would be ideal if the modal group was set up unconditionally. Unfortunately, I couldn't figure out a robust way to make that happen. Group factories are just callables, so `discr_tag_to_group_factory[DISCR_TAG_BASE].order` isn't guaranteed to work. We could try that (and give up if it doesn't work), which would make the situation incrementally better. (The previous version of the code unconditionally did that, so...) But it's not a bulletproof fix either. I would be OK with a PR that does this.

_Originally posted by @inducer in https://github.com/inducer/grudge/pull/329#discussion_r1515239388_