Fix `mapchunks` and write Test for it

While looking for a way to flatten an DArray, I found `mapchunks`, which looks perfectly suited for this job. Unfortunately it is currently buggy. Probably because there is no test for it.

https://github.com/JuliaParallel/Dagger.jl/blob/cec17a14f75523bdcae3a6b00ede6c22d1d0fe9f/src/array/darray.jl#L387-L393

The DArray constructor used here does not exist any longer. There is a new argument - the partitioning - which need to be added.

```diff
- DArray{T,N,F}(d.domain, d.subdomains, chunks, d.concat)
+ DArray{T,N}(d.domain, d.subdomains, chunks, d.partitioning, d.concat)
```