BlockBandedMatrix should have extra entries so subblocks are banded

[dlfivefifty]

Ideally we'd have:

julia> B = BlockBandedMatrix{Float64}(undef, Fill(2,10), Fill(3, 10), (1, 2));

julia> MemoryLayout(view(B, getindex.(Block.(1:10), 1), getindex.(Block.(1:1), 2))) isa BandedColumns{StridedLayout}

To do this, we have several things to do:

1. Support getindex.(Block.(1:10), 1). This will need a new type in BlockArrays.jl
2. Split out BlockBandedMatrix from BlockSkylineMatrix (to simplify things)
3. Special case when the block sizes are all the same size.