Removes nearly all warnings experienced with g++-8.3

Namely:
- unused parameter
- type qualifiers ignored ...
- catchinhg polymorphic type ... by value

examples/msim.cpp

@@ -30,7 +30,7 @@
 #include <opm/msim/msim.hpp>
 
 
-int main(int argc, char** argv) {
+int main(int /* argc */, char** argv) {
     std::string deck_file = argv[1];
     Opm::Parser parser;
     Opm::ParseContext parse_context;