The AbstractBlockArray interface

In order to follow the AbstractBlockArray the following methods should be implemented:

Methods to implement	Brief description
`nblocks(A)`	Tuple of number of blocks in each dimension
`nblocks(A, i)`	Number of blocks in dimension `i`
`blocksize(A, i...)`	Size of the block at block index `i...`
`getblock(A, i...)`	`X[Block(i...)]`, blocked indexing
`setblock!(A, v, i...)`	`X[Block(i...)] = v`, blocked index assignment
Optional methods
`getblock!(x, A, i)`	`X[i]`, blocked index assignment with in place storage in `x`

For a more thorough description of the methods see the public interface documentation.

With the methods above implemented the following are automatically provided:

A pretty printing show function that uses unicode lines to split up the blocks:

julia> A = BlockArray(rand(4, 5), [1,3], [2,3])
2×2-blocked 4×5 BlockArrays.BlockArray{Float64,2,Array{Float64,2}}:
0.61179   0.965631  │  0.696476   0.392796  0.712462
--------------------┼-------------------------------
0.620099  0.364706  │  0.0311643  0.27895   0.73477
0.215712  0.923602  │  0.279944   0.994497  0.383706
0.569955  0.754047  │  0.0190392  0.548297  0.687052

A bounds index checking function for indexing with blocks:

julia> blockcheckbounds(A, 5, 3)
ERROR: BlockBoundsError: attempt to access 2×2-blocked 4×5 BlockArrays.BlockArray{Float64,2,Array{Float64,2}} at block index [5,3]

Happy users who know how to use your new block array :)