Add flexible solver and preconditioner infrastructure.

Also use it in flow_blackoil_dunecpr.cpp. Adds new command-line parameter,
--linear-solver-configuration-json-file, to read linear solver config from
JSON-format file at runtime.

tests/rhs3.txt

@@ -0,0 +1,12 @@
+%%MatrixMarket matrix array real general
+% ISTL_STRUCT blocked 3 1
+9 1
+-4.48332e-07
+3.53746e-07
+4.31812e-05
+1.48051e-07
+-5.76325e-07
+-0.000243496
+-3.81652e-08
+1.21761e-06
+-9.97615e-05