par_for defaults

[pgrete]

          Actually a broader conversation... which maybe I should start and/or just implement, is the `par_for`s that aren't tied to meshblock shouldn't need to take in an exec space. We should provide overloads that accept default arguments. (Not your problem for this MR though.)

Originally posted by @Yurlungur in #1024 (comment)

[pgrete]

I'd even go as far as suggesting to remove the pmb->par_for entirely and replace it with a md->par_for (in addition to the default one).
My main motivation is that the original idea behind the pmb version was to be able to dispatch to different streams in parallel.
Given what we learned over the past years, I don't see us going back to the individual block level, but remain at the MeshData level and thus moving the pmb->par_for upwards in the hierarchy.

[pgrete]

From the sync https://github.com/parthenon-hpc-lab/parthenon/wiki/2024.03.28-Meeting-Notes

• Generic functionality is preferable (as it causes less confusion)
• For now, add shortened global interface
• Eventually deprecate the pmb->par_for
• Potentially introduce a md version (again as generic function that takes an md object) in the future