melt transport AMG constant mode settings are never used

[tjhei]

While looking at the code with @quangx we discovered that the forgot to use the AMG setting for constant_modes is computed but never used, see

aspect/source/simulator/assembly.cc

Lines 515 to 531 in 00b2507

	std::vector<std::vector<bool>> constant_modes;
	dealii::ComponentMask cm_pressure = introspection.component_masks.pressure;
	if (parameters.include_melt_transport)
	cm_pressure = cm_pressure | introspection.variable("compaction pressure").component_mask;
	DoFTools::extract_constant_modes (dof_handler,
	cm_pressure,
	constant_modes);
	Amg_data.elliptic = true;
	Amg_data.higher_order_elements = false;
	Amg_data.smoother_sweeps = 2;
	Amg_data.coarse_type = "symmetric Gauss-Seidel";
	LinearAlgebra::PreconditionAMG *Mp_preconditioner_AMG
	= dynamic_cast<LinearAlgebra::PreconditionAMG *> (Mp_preconditioner.get());
	Mp_preconditioner_AMG->initialize (system_preconditioner_matrix.block(1,1), Amg_data);

@quangx can you please prepare a PR to fix this so we can see the impact?

[gassmoeller]

Fixed by #6222.