Adding structured spmv

[lucbv]

I am proposing an implementation of spmv for structured matrices.
I am covering only simple stencils:

1D 3pts FE/FD stencil
2D 5pts FD and 9pts FE stencils
3D 7pts FD and 27pts FE stencils

The basic idea is to split the mesh into two parts: interior and exterior.
The interior is entirely structured and spmv can be accelerated using the know sparsity pattern if the matrix to access the entries of x without using the view of column indices of A.
The exterior is not necessarily fully structured due to boundary conditions and therefore is treated using the classic spmv algorithm.