treemesh: default diagonal balance = True

[lheagy]

As we talked about in the SimPEG meeting on June 5, we should set the default for mesh creation to do a diagonal balance to prompt best practices (cc @fourndo and @dccowan who prompted the discussion!)

[jcapriot]

Sounds great to me! Should just need to put in a notice on the init that the default has recently changed.

I believe it’s already set up that a serialized un-diagonal balanced mesh will still be read in consistently (for backwards compatibility), but that would be the only thing to double check.

[jcapriot]

I've added a note in #382 to emit a warning that this behavoir will change in discretize v1.0, that will start showing up when the bugfix release hits soon.