Add and use PreconditionerFactory class.

This replaces the makePreconditoner() function. The main advantage
is that it is extensible, making it easy to for example add new
preconditioners to the factory at runtime. It supports both parallel
and serial preconditioners.

tests/options_flexiblesolver_simple.json

@@ -0,0 +1,9 @@
+{
+    "tol": "1e-12",
+    "maxiter": "200",
+    "verbosity": "0",
+    "solver": "bicgstab",
+    "preconditioner": {
+        "type": "nothing"
+    }
+}