Determine index of pressure from model used.

Previously, the user had to specify it in the json file read from the
FlexibleSolver or 1 was used. Unfortunately, the index depends on the
model used and it seem rather opaque to a user what that index is.

With this commit we determine the pressure index from the model.

tests/options_flexiblesolver.json

@@ -25,8 +25,7 @@
             "solver": "bicgstab"
         },
         "verbosity": "11",
-        "weights_filename" : "weight_cpr.txt",
-        "pressure_var_index" : "1"
+        "weights_filename" : "weight_cpr.txt"
     },
     "verbosity": "10",
     "solver": "bicgstab"