ENH: warn user in case a Dataset subclass is redefined

[neutrinoceros]

PR Summary

Following a recent discussion on the Slack space in which we discussed how yt extensions
provided support to create third party frontends, which allows users to extend over native one as

import yt
import yt.extensions.my_frontend

The only issue I was able to see with this workflow is that, in case such a frontend gets integrated
into yt itself, then users may not realize that the extension import became useless, or worse. Shadowing a "first citizen" frontend may actually be useful as temporary measure, which is why I'm not making this an error, but it seems fair to emit a warning as a forgiving gesture to transiting users.

PR Checklist

[N/A] New features are documented, with docstrings and narrative docs

• Adds a test for any bugs fixed. Adds tests for new features.

note : I want to add documentation for the whole extension frontend workflow later, but I think this small patch counts as its own documentation.

[brittonsmith]

This seems like a reasonable solution to me. I wouldn't want to prevent an extension over-writing an internal frontend, but a warning to make it clear what's happening is good.

[neutrinoceros]

@Xarthisius @brittonsmith, any reason not to merge this ?