Update darray.jl

[raminammour]

Closes #162 , eltype should infer correctly to the promoted part of all the types.
The original implementation assumed the same type from all the pids.

[vchuravy]

Thank you for looking into this!

So it is possible to construct a DArray where the localparts have a different eltype? I think using promote_eltype is the wrong solution for that, since the DArray now diverges from the eltype of its localparts, those are assumed to be the same in many places and I am hesitant to remove that invariant. I rather would error in this case and try very hard to not construct localparts of differing eltype.

[raminammour]

Weirdly enough, this change does not break any of the tests :)

I understand if you want to preserve this invariant and want to make into an error...

[vchuravy]

Thanks! But yes I would prefer this being an error.

[vchuravy]

Thanks Ramin, for your work. I finished the PR in #170