Generalize localpart

To make it easier to see the `localpart`s of a `DArray`, introduce a two argument version `localpart(d::DArray, p = myid())` that returns the `localpart` located at process `p` as an array on the calling process. When `p==myid()` the function will work as it does now, i.e. no communication happens.

cc: @alanedelman

Start by locating the existing localpart implementation for DArray and the code defining process ownership and myid(). Trace how the current localpart is handled when p is the calling process, then determine the expected behavior for another process. Done means localpart(d, p) returns the selected localpart on the caller and preserves the no-communication behavior when p == myid().

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