Higher dimensional concretizations

It would be nice for the following to work:

```julia
using DiffEqOperators
N = 64
dx = 0.1
Dxx = CenteredDifference(2,2,dx,N)
Dyy = CenteredDifference{2}(2,2,dx,N)
L = Dxx + Dyy
using BlockBandedMatrices
BlockBandedMatrix(L)
```

Xref: https://discourse.julialang.org/t/best-way-to-construct-2-d-laplacian-banded-matrix-bandedmatrices-sparse-or-blockbandedmatrices/29123