Numpy decided to deprecacte multi-indexing with anything other than a tuple.

regular grid currently returns a list. It might be useful to keep up with numpy's API, and as such, we should return a tuple.

For example, just below in _regular_grid.py

uses the output directly to index a numpy array.

Unfortunately, lists are not tuples, and this would cause a breaking change in the (internal) API.

How should we proceed with this?

Ideas I have:

1. Check the numpy version, if greater than 1.15 then we can return a tuple.
2. Return a list until numpy decides to change their implementation
3. Return a tuple instead of a list.

I kinda like 3, but doesn't seem prudent.

Edit: it seems that this is actually an external facing API.