fix the parallel AMG linear solver

... and actually use it for the lens problems. This seems to have been
disabled for debugging and later it was probably forgotten to turn it
on again. This led to some minor bit-rot in that code...

examples/problems/lensproblem.hh

@@ -110,7 +110,7 @@ public:
 };
 
 // Use the algebraic multi-grid linear solver for this problem
-SET_TAG_PROP(LensBaseProblem, LinearSolverSplice, ParallelAmgBackend);
+SET_TAG_PROP(LensBaseProblem, LinearSolverSplice, ParallelAmgLinearSolver);
 
 // Write the solutions of individual newton iterations?
 SET_BOOL_PROP(LensBaseProblem, NewtonWriteConvergence, false);