include iostream in the files which use std::cerr or std::cout

for some of these files this is needed to make to keep it compiling
after the next patch because the new ErrorMacros.hpp file will no
longer implicitly includes <iostream>. for the remaining files it is
just good style.

While at it, the includes for most of these files have been ordered in
order of decreasing abstraction level.

opm/simulators/SimulatorCompressibleTwophase.cpp

@@ -54,6 +54,7 @@
 
 #include <numeric>
 #include <fstream>
+#include <iostream>
 
 
 namespace Opm

opm/simulators/SimulatorIncompTwophase.cpp

@@ -50,6 +50,7 @@
 #include <boost/filesystem.hpp>
 #include <memory>
 
+#include <iostream>
 #include <numeric>
 #include <fstream>
 

opm/simulators/SimulatorIncompTwophase.hpp

@@ -22,6 +22,7 @@
 
 #include <memory>
 #include <vector>
+#include <iostream>
 
 struct UnstructuredGrid;
 struct Wells;