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.
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.
