SparseAxisArray does not implement size

[matbesancon]

As an AbstractArray, size should be defined for SparseAxisArray and return an N-sized tuple.
The error is raised when collecting a SparseAxisArray for example.

MethodError: no method matching size(::JuMP.Containers.SparseAxisArray{Float64,2,Tuple{Any,Any}})

[odow]

So I implemented size for SparseAxisArray, but all manner of weird things arise:

julia> x = Containers.@container([i=1:4, j=i:3], i + j)
SparseAxisArray{Int64, 2, Tuple{Int64, Int64}} with 6 entries:
  [1, 1]  =  2
  [1, 2]  =  3
  [1, 3]  =  4
  [2, 2]  =  4
  [2, 3]  =  5
  [3, 3]  =  6

julia> size(x)
(3, 3)

julia> collect(x)
3×3 Matrix{Int64}:
 3  4  4511699696
 2  4  4511699760
 6  5  4511699824

Given how widespread some of the assumptions around size are, I think we're better off leaving it un-implemented. I'll push a PR that throws a nicer error to say that this is an explicit design choice.

[matbesancon]

In that case, I'd be in favor of dropping the subtyping to AbstractArray

[odow]

Yeah related is #2176.