Add firstindex(::AbstractDataContainer)

This enables the `begin` keyword when indexing an
`AbstractDataContainer`. Note that `end` already worked when indexing
because of `lastindex(::AbstractDataContainer)` is already implemented. For example:

```julia
julia> using MLDatasets

julia> mnist = MNIST()

julia> x, y = mnist[begin]
(features = Float32[0.0 0.0 … 0.0 0.0; 0.0 0.0 … 0.0 0.0; … ; 0.0 0.0 … 0.0 0.0; 0.0 0.0 … 0.0 0.0], targets = 5)
```

src/observation.jl

@@ -142,6 +142,7 @@ Base.size(x::AbstractDataContainer) = (numobs(x),)
 Base.iterate(x::AbstractDataContainer, state = 1) =
     (state > numobs(x)) ? nothing : (getobs(x, state), state + 1)
 Base.lastindex(x::AbstractDataContainer) = numobs(x)
+Base.firstindex(::AbstractDataContainer) = 1
 
 # --------------------------------------------------------------------
 # Arrays