Add ability to iterate on axis elements

[tlnagy]

It would be great to have the ability to do the following:

Main> a = AxisArray(randn(10,10,5), :x, :y, :time);

Main> timeaxis(a)
AxisArrays.Axis{:time,Base.OneTo{Int64}}(Base.OneTo(5))

Main> for timepoint in timeaxis(a)
          println(timepoint)
       end
ERROR: MethodError: no method matching start(::AxisArrays.Axis{:time,Base.OneTo{Int64}})
Closest candidates are:
  start(::SimpleVector) at essentials.jl:258
  start(::Base.MethodList) at reflection.jl:560
  start(::ExponentialBackOff) at error.jl:107
  ...
Stacktrace:
 [1] anonymous at ./<missing>:?
 [2] eval(::Module, ::Any) at ./boot.jl:235

That would be the most intuitive method for me to iterate over slices along an axis.

My current solution is way uglier and more fragile

for timepoint in 1:size(exp_img)[axisdim(exp_img, Axis{:time}())]
    println(timepoint)
end

[tlnagy]

Thoughts @Evizero @timholy?

[Evizero]

Sorry but I am not that involved with AxisArrays

[tlnagy]

I think this was resolved by #160