ENH: make printoptions a true context var

[mtsokol]

Proposed new feature or change:

Hi!

One of possible enhancements is to make printoptions use a context var. Right now it uses a _format_options dict defined in the arrayprint module - context var will give context-local state.

WDYT?

[ngoldbaum]

Yikes! I guess the current implementation isn't thread-safe then.

Do we want to allow thread-local print options? That seems strange to me. Better to make it like the errstate and make it a global setting that can only be entered once.

[seberg]

Heh, I thought there was an old issue about this somewhere, but didn't find it. In any case: any context manager should be backed by one (or maybe more) context variables to make sure it is truly context safe. And I would expect that to make the code a bit nicer as well.