Given a two-dimensional array of integers, return the middle element if both the row and column lengths are odd, or the average of the two or four middle elements if one or both is even. For instance:

{{1,2,3},{4,5,6},{7,8,9}} // height & width are odd; return 5.0
{{1,2,3,4},{4,5,6,7},{7,8,9,10}} // height odd, width even; return avg 5 + 6 or 5.5
{{1,2,3,4},{5,6,7,8},{9,10,11,12},{13,14,15,16}} // height & width even