Implements the Python step_init() function.

A resubmission of commit 11eaa3d7 in PR #2403 and PR #2443 and continues
the work in #2555 implementing Python bindings to the flow simulator.

The step_init() method initializes the simulation. It is required for the
Python script to run step_init() before calling the step() method (which
will be implemented in a later commit).

opm/simulators/flow/Main.hpp

@@ -119,6 +119,7 @@ namespace Opm
     {
     private:
         using FlowMainEbosType = Opm::FlowMainEbos<TTAG(EclFlowProblem)>;
+
         enum class FileOutputMode {
             //! \brief No output to files.
             OUTPUT_NONE = 0,