Add `Parthenon` frontend

[pgrete]

PR Summary

Add yt frontend for codes based on the Parthenon AMR framework (https://github.com/parthenon-hpc-lab/parthenon) including AthenaPK (https://github.com/parthenon-hpc-lab/athenapk), Phoebus (https://github.com/lanl/phoebus) or KHARMA (https://github.com/AFD-Illinois/kharma) Note that the mesh structure in Parthenon is heavily derived from Athena++, so that's why we used the Athena++ frontend as baseline.

We've been using this frontend for quite some time now (without any recent major changes) so we consider it stable enough to open it for review and merge to yt proper.

At this point I'm looking for

• general feedback
• feedback on how to generally handle/differentiate between Parthenon (as framework) and downstream code related functions/fields/... (the current version of this frontend is geared towards AthenaPK and I'd like to abstract this)
• (assuming a test dataset is desired), how to upload/handle/provide the dataset

PR Checklist

• [ ] New features are documented, with docstrings and narrative docs
• [ ] Adds a test for any bugs fixed. Adds tests for new features.