Feature/dist array proxy indexing

Add simple DistArrayProxy indexing.

More specifically, add a `__getitem__` and `__setitem__` to `DistArrayProxy`.

These are implemented in the style of previous `DistArrayProxy` functionality in that everything is deferred to underlying `DistArray`s.  In this version, all engines are queried for an index, and the ones that don't have it return `None`.