Add `diagonal` property to `KroneckerStencilMatrix`

[yguclu]

Given an N-dimensional KroneckerStencilMatrix, we want to construct its diagonal as an N-dimensional StencilDiagonalMatrix. To this end we will call the diagonal property of each 1D StencilMatrix, which returns a 1D StencilDiagonalMatrix, and use these N objects to create the N-dimensional StencilDiagonalMatrix.

Notes:

• 1D StencilDiagonalMatrix objects are not distributed, while the N-dimensional StencilDiagonalMatrix is.
• Pyccel-izable kernels should be added if necessary.