Replace direct access of tzone attribute

[joshuaulrich]

There are a handful of places where you directly access the "tzone" attribute from the xts object. This attribute is being removed from the xts object, in favor of using the "tzone" attribute on the xts index attribute. That means calls to attr(x, "tzone") will return NULL, which will likely cause errors.

I see you currently depend on xts >= 0.8-8. That version created xts objects that had a "tzone" attribute on the index, as well as on the xts object itself. That means you should be able to replace all your calls to attr(x, "tzone") with attr(.index(x), "tzone"). That still accesses the attribute directly, instead of using the tzone() accessor function. But xts::tzone() doesn't exist until after version 0.11-2...

So, a better and more robust solution would involve defining a new function in spacetime that would mimic xts::tzone() if xts <= 0.11-2 is installed, and use xts::tzone() for xts versions > 0.11-2. I could write this function and submit a PR, if you would like to go this route.

[edzer]

That would be great, Joshua!