Deprecation of Panel4D/PanelND
Related to https://github.com/pydata/pandas/issues/13563 (deprecation of Panels), we could/should also deprecate Panel4D / PanelND. 
(xref https://github.com/pydata/pandas/issues/5049)

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