fix some masochistic compiler warnings for the GCC 9 pre-release

the flags which I used are
```
-pedantic \
-Wall \
-Wextra \
-Wformat-nonliteral \
-Wcast-align
-Wpointer-arith \
-Wmissing-declarations \
-Wcast-qual \
-Wshadow
-Wwrite-strings \
-Wchar-subscripts \
-Wredundant-decls \
-fstrict-overflow \
-O3 \
-march=native \
-DNDEBUG=1
```

note that some heavy filtering is not the worst idea because DUNE is
far from not emiting any warnings with these flags.

Also, there were some pesky warnings in test_ecl_output which I don't
know how to fix:

```
tests/test_ecl_output.cc:218:73: warning: missing initializer for member ‘Opm::data::Connection::effective_Kh’ [-Wmissing-field-initializers]
```

ebos/eclwellmanager.hh

@@ -101,7 +101,7 @@ public:
      *
      * I.e., well positions, names etc...
      */
-    void init(const Opm::EclipseState& eclState,
+    void init(const Opm::EclipseState& eclState OPM_UNUSED,
               const Opm::Schedule& deckSchedule)
     {
         // create the wells which intersect with the current process' grid
@@ -609,7 +609,7 @@ public:
     }
 
 protected:
-    bool wellTopologyChanged_(const Opm::EclipseState& eclState,
+    bool wellTopologyChanged_(const Opm::EclipseState& eclState OPM_UNUSED,
                               const Opm::Schedule& schedule,
                               unsigned reportStepIdx) const
     {