[example][1ptracer] Lower solver requirements (only use dune-istl)

(cherry picked from commit 884677ab)

examples/1ptracer/CMakeLists.txt

@@ -3,7 +3,6 @@ dune_symlink_to_source_files(FILES "params.input")
 dumux_add_test(NAME example_1ptracer
                LABELS porousmediumflow tracer example
                SOURCES main.cc
-               CMAKE_GUARD HAVE_UMFPACK
                COMMAND ${CMAKE_SOURCE_DIR}/bin/testing/runtest.py
                CMD_ARGS --script fuzzy
                         --files ${CMAKE_SOURCE_DIR}/test/references/test_1ptracer_transport-reference.vtu

examples/1ptracer/doc/main.md

@@ -161,7 +161,8 @@ The grid variables are used store variables (primary and secondary variables) on
 ```
 
 We now instantiate the assembler class, assemble the linear system and solve it with the linear
-solver UMFPack. Besides that, the time needed for assembly and solve is measured and printed.
+solver ILUnBiCGSTABBackend (a bi-conjugate gradient solver preconditioned by an incomplete LU-factorization preconditioner).
+Besides that, the time needed for assembly and solve is measured and printed.
 
 ```cpp
     using OnePAssembler = FVAssembler<OnePTypeTag, DiffMethod::analytic>;
@@ -175,7 +176,7 @@ solver UMFPack. Besides that, the time needed for assembly and solve is measured
 
     (*r) *= -1.0; // We want to solve `Ax = -r`.
 
-    using LinearSolver = UMFPackBackend;
+    using LinearSolver = ILUnBiCGSTABBackend;
     Dune::Timer solverTimer; std::cout << "Solving linear system ..." << std::flush;
     auto linearSolver = std::make_shared<LinearSolver>();
     linearSolver->solve(*A, p, *r);

examples/1ptracer/main.cc

@@ -127,7 +127,8 @@ int main(int argc, char** argv) try
     onePGridVariables->init(p);
 
     // We now instantiate the assembler class, assemble the linear system and solve it with the linear
-    // solver UMFPack. Besides that, the time needed for assembly and solve is measured and printed.
+    // solver ILUnBiCGSTABBackend (a bi-conjugate gradient solver preconditioned by an incomplete LU-factorization preconditioner).
+    // Besides that, the time needed for assembly and solve is measured and printed.
     using OnePAssembler = FVAssembler<OnePTypeTag, DiffMethod::analytic>;
     auto assemblerOneP = std::make_shared<OnePAssembler>(problemOneP, gridGeometry, onePGridVariables);
     assemblerOneP->setLinearSystem(A, r); // tell assembler to use our previously defined system
@@ -139,7 +140,7 @@ int main(int argc, char** argv) try
 
     (*r) *= -1.0; // We want to solve `Ax = -r`.
 
-    using LinearSolver = UMFPackBackend;
+    using LinearSolver = ILUnBiCGSTABBackend;
     Dune::Timer solverTimer; std::cout << "Solving linear system ..." << std::flush;
     auto linearSolver = std::make_shared<LinearSolver>();
     linearSolver->solve(*A, p, *r);