[Reactor] Use right preconditioning by default

Using right preconditioning provides a more reliable performance benefit
even after the ignition event, allowing the solver to take reasonable
timesteps and showing a CPU-time benefit even for a single reactor with
the 100-species n-dodecane mechanism.

samples/python/reactors/preconditioned_integration.py

@@ -11,7 +11,7 @@
 import matplotlib.pyplot as plt
 from timeit import default_timer
 
-def integrate_reactor(n_reactors=15, preconditioner=True):
+def integrate_reactor(n_reactors=1, preconditioner=True):
     """
         Compare the integrations of a preconditioned reactor network and a
         non-preconditioned reactor network. Increase the number of reactors in the

src/numerics/AdaptivePreconditioner.cpp

@@ -11,7 +11,7 @@ namespace Cantera
 
 AdaptivePreconditioner::AdaptivePreconditioner()
 {
-    setPreconditionerSide("left");
+    setPreconditionerSide("right");
 }
 
 void AdaptivePreconditioner::setValue(size_t row, size_t col, double value)

test/python/test_reactor.py

@@ -1042,11 +1042,13 @@ class TestIdealGasConstPressureMoleReactor(TestIdealGasConstPressureReactor):
 
     def create_reactors(self, **kwargs):
         super().create_reactors(**kwargs)
-        self.net2.preconditioner = ct.AdaptivePreconditioner()
+        self.precon = ct.AdaptivePreconditioner()
+        self.net2.preconditioner = self.precon
         self.net2.derivative_settings = {"skip-third-bodies":True, "skip-falloff":True}
 
     def test_get_solver_type(self):
         self.create_reactors()
+        assert self.precon.side == "right"
         self.assertEqual(self.net2.linear_solver_type, "GMRES")
 
     def test_with_surface_reactions(self):