Allow construction of interpolation matrix between a VertexOnlyMesh and its input-ordering

[leo-collins]

Description

Allows construction of the interpolation matrix between a VOM and its input-ordering. This is a permutation matrix which is used for cross-mesh interpolation to redistribute the points correctly on the target mesh.

Set the keyword argument matfree=False to construct the matrix. By default matfree=True which uses the current way of SF reduce and broadcasts.

P0DG = FunctionSpace(vom, "DG", 0)
P0DG_io = FunctionSpace(vom.input_ordering, "DG", 0)
B = assemble(interpolate(TestFunction(P0DG), P0DG_io, matfree=False))