Operations that call `collect` or `iterate` fail on some RasterStacks

[asinghvi17]

Here's a running example (from my mapzonal code):

Setup:

using Rasters

r1 = Raster(rand(X(1:10), Y(1:10)))
r2 = Raster(rand(dims(r1)...))

rs = RasterStack(r1, r2)

bool_cond_raster = r1 .> 0.5

mask_inds = view(Rasters.DD.DimIndices(bool_cond_raster), bool_cond_raster)

masked_stack = rs[mask_inds] # fine so far

Operations on a directly masked stack

masked_stack[1] # also fine

collect(masked_stack) # errors

identity.(masked_stack) # errors
sum(masked_stack) # somehow fine and correct?

# errors for anything that calls `collect

Operations on a viewed stack

# If I try to view,

viewed_stack = view(rs, mask_inds)

viewed_stack[1] # also fine

collect(viewed_stack) # also errors

sum(viewed_stack) # also fine and correct

The error:

ERROR: Use iterate(layers(s)) rather than `iterate(s)`
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] iterate(::RasterStack{…})
   @ DimensionalData ~/.julia/dev/DimensionalData/src/stack/stack.jl:169
 [3] copyto!(dest::Vector{…}, src::RasterStack{…})
   @ Base ./abstractarray.jl:943
 [4] _collect(cont::UnitRange{…}, itr::RasterStack{…}, ::Base.HasEltype, isz::Base.HasLength)
   @ Base ./array.jl:765
 [5] collect(itr::RasterStack{…})
   @ Base ./array.jl:759
 [6] broadcastable(x::RasterStack{…})
   @ Base.Broadcast ./broadcast.jl:743
 [7] broadcasted(::typeof(identity), ::RasterStack{…})
   @ Base.Broadcast ./broadcast.jl:1339
 [8] top-level scope
   @ REPL[99]:1
Some type information was truncated. Use `show(err)` to see complete types.

```1

[rafaqz]

Yeah, we need to define what iterate on a stack does and I never settled. It's like base with broadcast over a Dict. The fact that stack layers can have mixed sizes complicates things a little.

Edit: I already made the old version error, we can fix it now.