SIGBUS in db::create_from on macOS

[nilsvu]

Bug reports:

I get a bus error when calling db::create_from with a large amount of simple tags, and running in parallel on 2 or more processors. Splitting the call in two resolves the issue. I haven't investigated this issue any further than this, but noting it down here in case anyone else runs into this problem.

Edit: This seems to be an issue only on macOS.

[nilsvu]

@kidder and I traced this through the DataBox code. This is what we found:

• The bus error might be caused by a stack overflow. @kidder found that the stack size is smaller on extra threads (hence triggering the bus error when running on multiple threads) and could also trigger the bus error on a single thread by reducing its stack size.

• The offending lines seem to be this:

  https://github.com/sxs-collaboration/spectre/blob/537c7da879bbb879ddd95f1b2ede15344142a5df/src/DataStructures/DataBox/DataBox.hpp#L736-L739

  Removing the body of the add_items_to_box and also removing its meta arguments didn't help, neither did any variation of forwarding the arguments as a tuple of rvalue refs and taking the tuple itself by rvalue ref or by value. Perhaps such a large tuple of rvalue refs just overflows the stack, though I would find that very odd 🤷‍♂️

• To work around the issue, we could call add_items_to_box in chunks of a reasonable number of tags. Since it's probably very rare that db::create_from needs to be called with a huge amount of tags that each should be assign an actual value, another workaround is to omit the call to add_items_to_box altogether when all tags should just be default-constructed. We just want to default-construct the tags anyway in the one situation where I hit this bus error, i.e. in SetupDataBox, so I suggest the latter approach (here https://github.com/sxs-collaboration/spectre/pull/2899#discussion_r579469906).

[geoffrey4444]

On my Apple Silicon Mac, I get a bus error when trying to run SolveXcts on more than one core. Specifically, the error happens when trying to call invoke_iterable_action on the action Actions::SetupDataBox, which does have a call to db::create_from inside it