Add gather(d::DArray{T,N,Array{T,N}}) where {T, N}

I am wondering why it is not possible to gather a DArray{T,N,Array{T,N}}?

I wrote this method to the gather function, it works for my purpose. Maybe it is possible to add it? (or maybe I did a mistake and it should be changed somehow?)
```
function gather(d::DArray{T,N,Array{T,N}}) where {T, N}
a=Array{T}(undef, size(d))
indices = collect.(d.indices)
@sync for (i,p) in enumerate(procs(d))
@async a[indices[i]...] = remotecall_fetch(localpart, p, d)
end
a
end
```

首先定位 DistributedArrays.jl 中现有的 gather 函数和 DArray 定义，然后将当前的 gather 方法与建议的 DArray{T,N,Array{T,N}} 签名进行比较。使用本地部分采用 Array 存储的分布式数组验证其行为；完成的标准是 gather 返回预期的、填充了所有分布式索引的 Array。

由索引模型根据 Issue 内容生成。