`selectdim` for DimStack

[Beforerr]

da = rand(X(1:10), Y(1:10))
ds = DimStack((a = da, b = da))
ds[1:3,:] # working
@view ds[1:3,:] # working
selectdim(ds,1,1:3) # not working

Maybe it would be nice to make selectdim work?

[rafaqz]

I guess it should also work with dimensions for Dim, and selectors as the last argument.

[rafaqz]

But also, what do you use this for? isn't it more concise to do ds[X=1:3] ?

[Beforerr]

Yes, ds[X=1:3] is more concise and better if we know ds isa DimStack.

My use case is that I have generic code do selectdim on array-like objects. Since DimStack is kinda of like an array, having selectdim work would simplify the logic and codes.

[rafaqz]

Makes total sense. Try a PR if you are up for it, I am very short on time currently.