Selective algebraic multigrid in foam-extend

A newly implemented algebraic multigrid (AMG) algorithm based on the classical selective method (SAMG) is presented. It is compared to AAMG, aggregation based algebraic multigrid which can be considered a special case of SAMG. In SAMG, interpolation operators are calculated based on the connectivity pattern of the matrix, while in AAMG interpolation is done by injection. More accurate interpolation provides better convergence, especially with a tight residual tolerance.