REPL show of DArray makes 4 round trip calls for each element.

I noticed that the default `show` of `DArray` (i.e., it defaults to the one in Base) results in 4 round trip calls for each element. We ought to define ` Base.show(io::IO, mime::MIME"text/plain", D::DArray)` in this package.

The question is should it
1) Only print DArray meta data (which worker has which indexes and so on) or
2) Efficiently call a `convert(Array, D)` and then print it in one go locally or
3) Pull in and show only the first few and last few elements of the darray.

Start by examining the default Base.show behavior for DArray and how many round trips the current REPL display causes. Compare the three output approaches listed in the issue, then establish which behavior should be selected before defining Base.show(io::IO, MIME"text/plain", D::DArray). Done means the chosen representation displays without four round trips per element.

Vom Indexierungsmodell aus dem Issue-Text verfasst.