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]
```

examples/lens_immiscible_ecfv_ad_cu1.cpp

@@ -38,7 +38,9 @@
 
 #include <ewoms/common/start.hh>
 
+// fake forward declaration to prevent esoteric compiler warning
 int mainCU1(int argc, char **argv);
+
 int mainCU1(int argc, char **argv)
 {
     typedef TTAG(LensProblemEcfvAd) ProblemTypeTag;

examples/lens_immiscible_ecfv_ad_cu2.cpp

@@ -38,6 +38,9 @@
 
 #include <ewoms/common/start.hh>
 
+// fake forward declaration to prevent esoteric compiler warning
+int mainCU2(int argc, char **argv);
+
 int mainCU2(int argc, char **argv)
 {
     typedef TTAG(LensProblemEcfvAd) ProblemTypeTag;