Ambiguous field/particle type: special pixel-fields

[neutrinoceros]

Bug report

Bug summary

Right now when running for instance

pytest yt/frontends/ytdata/tests/test_unit.py

one gets deprecation warnings matching the following regexp

r"The requested field name 'pd?[xyz]' is ambiguous and corresponds to any one of the following field types.*"

This is because internally, slicing and coordinate handler classes are using some special, pixel-related "fields" "px", "py", "pz", "pdx", "pdy" and "pdz", but leave the field (particle) type unspecified.
There doesn't seem to be an obvious path to follow here to resolve the warnings, that would work with every kind of dataset (grid/particles). Ideally we should be able to determine the appropriate type at runtime but this is currently not trivial according to discussions with @cphyc.
This will need to be resolved somehow in order to fully remove compatibility with ambiguous fields, as is targeted for yt 4.1.0

Right now I'm going to ignore this warning in #3380, but I'll reference this issue in the code (specifically in conftest.py) to keep track of it.

[neutrinoceros]

I think this specific problem is another aspect of #3839