> I appreciate the context manager is cool, but... Can we highlight the ability to simply set... and advise where users should put this?

I get the point, but the example in the userguide **does** use the context manager, which I chose because..
  * it generally looks neater so is easier to read
  * it's pretty clear what it does, and you can look up iris.FUTURE to check it
  * I think it's good to generally encourage the use of context managers.  

I also found there that if you just 'turn it on' then other doctests will fail.  So you need to cancel it too, which looks ugly.  Possibly that can be fixed with a '..testsetup' *after* the code?  but I don't know if that actually works.