refactor(mp7particledata): match mp7 order in to_coords()/to_prp()

[wpbonelli]

• return the same particle order from to_coords() and to_prp() as generated by MP7 — preserves particle indexing for MF6 PRT, making it easier to compare results, though something like this may still be handy
• refactoring/cleanup, factor out helper methods get_cell_release_points(), get_face_release_points()
  • defer materialization — return iterators not lists so the consumer can control memory use, previously done for to_coords() and to_prp() but not get_release_points(), might be relevant for very large release configurations
• test-drive a pytest plugin for previously hardcoded snapshot tests, introduce some fixtures in autotest/conftest.py for nicer array snapshots than the default format, maybe these could live in devtools if they see wider use?

[wpbonelli]

Changes here should only affect PRT, I will merge aggressively since @aprovost-usgs and I are dogfooding this and the tentative snapshot approach has been discussed offline with @langevin-usgs. Note, contributors will need to install the plugin