maint: cleanup usage and mentions of deprecated material

[neutrinoceros]

PR Summary

I discovered some deprecated functionalities are still used internally or documented as usable while they really aren't (add_volume_weighted_smoothed_field)

[neutrinoceros]

Failures on Jenkins are real. Some of the answer tests' names contains the name of a deprecated function set_window_size, so the newly named generated tests don't match any existing answer.
I read the docs in search for a solution and found
https://yt-project.org/doc/developing/testing.html#enabling-answer-tests-on-jenkins

If the answer doesn’t exist on the server yet, nosetests is run twice and during first pass --answer-store is added to the commandline.

which doesn't seem appropriate.
I'll try bumping the answers and hope it'll trigger regeneration.

update: yay, it worked !