Distributed linear solvers

So this is probably ready - please review and criticise 😉

CC @oberhuber @hanousek

src/TNL/Matrices/SparseOperations.h

+/***************************************************************************
+                          SparseOperations.h  -  description
+                             -------------------
+    begin                : Oct 4, 2018
+    copyright            : (C) 2018 by Tomas Oberhuber et al.
+    email                : tomas.oberhuber@fjfi.cvut.cz
+ ***************************************************************************/
+
+/* See Copyright Notice in tnl/Copyright */
+
+// Implemented by: Jakub Klinkovský
+
+// Note that these functions cannot be methods of the Sparse class, because
+// their implementation uses methods (marked with __cuda_callable__) which are
+// defined only on the subclasses, but are not virtual methods of Sparse.
+
+#pragma once
+
+namespace TNL {
+namespace Matrices {
+
+template< typename Matrix1, typename Matrix2 >
+void copySparseMatrix( Matrix1& A, const Matrix2& B );
+
+// NOTE: if `has_symmetric_pattern`, the sparsity pattern of `A` is assumed
+// to be symmetric and it is just copied to `B`. Otherwise, the sparsity
+// pattern of `A^T + A` is copied to `B`.
+template< typename Matrix, typename AdjacencyMatrix >
+void
+copyAdjacencyStructure( const Matrix& A, AdjacencyMatrix& B,
+                        bool has_symmetric_pattern = false,
+                        bool ignore_diagonal = true );
+
+// Applies a permutation to the rows of a sparse matrix and its inverse
+// permutation to the columns of the matrix, i.e. A_perm = P*A*P^{-1}, where
+// P is the permutation matrix represented by the perm vector and P^{-1} is the
+// inverse permutation represented by the iperm vector.
+template< typename Matrix1, typename Matrix2, typename PermutationVector >
+void
+reorderSparseMatrix( const Matrix1& A, Matrix2& A_perm,
+                     const PermutationVector& perm, const PermutationVector& iperm );
+
+// TODO: the method does not belong here, but there is no better place...
+template< typename Vector, typename PermutationVector >
+void
+reorderVector( const Vector& src, Vector& dest, const PermutationVector& perm );
+
+} // namespace Matrices
+} // namespace TNL
+
+#include "SparseOperations_impl.h"