Output Non-Linear Convergence to Separate File if Requested

This commit enables outputting non-linear convergence metrics, i.e.,
the MB and CNV values, per phase, for each non-linear iteration in
each timestep.  If the user passes the option value "iterations" to
the --extra-convergence-output command line option, this commit will
create a new output file, CASE.INFOITER, that holds

    * report step
    * time step within that report step
    * elapsed time
    * MB and CNV values per phase
    * well convergence status

for each non-linear iteration.

We use an asynchronous file writing procedure and confer ownership
of the report step's unprocessed convergence reports to this
procedure just before the end of

    SimulatorFullyImplicitBlackoilEbos::runStep()

At that point, the convergence reports are about to go out of scope.
The asynchronous protocol uses a dedicated queue of output requests,
class ConvergenceReportQueue, into which the producer-i.e., member
function runStep()-inserts new convergence reports and from which
the output thread, ConvergenceOutputThread::writeASynchronous(),
retrieves those requests before writing the file data.

tests/test_convergenceoutputconfiguration.cpp

@@ -135,6 +135,9 @@ BOOST_AUTO_TEST_CASE(Misprint)
     BOOST_CHECK_THROW(Opm::ConvergenceOutputConfiguration{"nonce"},
                       std::invalid_argument);
 
+    BOOST_CHECK_THROW(Opm::ConvergenceOutputConfiguration("nonce", "X"),
+                      std::invalid_argument);
+
     BOOST_CHECK_THROW(Opm::ConvergenceOutputConfiguration{"stepS"},
                       std::invalid_argument);