Added Solver Monitor interface

src/system/SolverMonitor.h

+// GmshFEM - Copyright (C) 2023, B. Martin, C. Geuzaine, Université de Liège
+//
+// See the LICENSE.txt file for license information. Please report all
+// issues on https://gitlab.onelab.info/gmsh/fem/issues
+
+#ifndef H_GMSHFEM_SOLVER_MONITOR
+#define H_GMSHFEM_SOLVER_MONITOR
+
+typedef struct _p_KSP *KSP;
+
+#ifdef HAVE_PETSC
+#include "petscksp.h"
+#endif
+
+namespace gmshfem::system
+{
+  struct AbstractMonitor {
+#ifdef HAVE_PETSC
+    virtual PetscErrorCode Monitor(KSP ksp, PetscInt n, PetscReal rnorm) = 0;
+#endif
+    virtual ~AbstractMonitor() = default;
+  };
+
+} // namespace gmshfem::system
+
+#endif
\ No newline at end of file