genbox: add some warning when using periodic

[yslan]

1. periodic + nelx < 3
   It's a hidden rule that you need 3 layers to make periodic BC working in Nek (without manually fixing the vertices id like uservert).
   However, this information is not easily found.

   We should at least put a warning in genbox (or even prenek) to let user know.

2. periodic + multiple boxes
   The auto periodic will fail if there are multiple boxes placed in the periodic direction.
   User will have to fix it with other tools like prenek while genbox should give some instruction or warning.

And maybe put a link redirecting user to nekdoc would be nice.

[stgeke]

@yslan Can you please add a PR for this.