Fix `mapchunks` and write Test for it

[schlichtanders]

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.

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

[schlichtanders]

Furthermore, it would be great if

• mapchunks also supports EagerThunk
• mapchunks handles the new DArray type T correctly. (By inspecting the mapped chunks, or falling back to type-inference or Any, if there are no chunks)