Hdf warning

[dacoex]

In accordance with #12268 this is now a separate PR.

I think that this warning could be useful either way for no non-daily hdf user:

• Telling that string method is preferred
• Adding a warning

[jreback]

@dacoex idea being that you separate the changes into 2 PRs, one containing the pd. changes, the other the doc-changes w.r.t. hierarchical keys. (there are 2 PR's now, but they both contain all the changes)

[dacoex]

Sorry, my git skills do not make it.
I will update them during the next days.

[jreback]

hahha, no prob. you will want to rebase on master, then delete that commit.

e.g.

git rebase -i master

then force push

[jreback]

pls rebase on master

[dacoex]

I hope I got it right now ;-)

[dacoex]

anything holding this one back?

[jreback]

thanks!