Deprecation of Panel4D/PanelND

[jorisvandenbossche]

Related to #13563 (deprecation of Panels), we could/should also deprecate Panel4D / PanelND.
(xref #5049)

Deprecating Panel4D/ND could probably be done sooner than deprecating Panels (e.g. actually deprecating in 0.19.0 ?)

[wesm]

I'm very in favor of this. I will leave some comments on #13563

[jorisvandenbossche]

Let's do this in 0.19.0 then