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]
```
this is similar to the mechanism for well models: the API is defined
here, but can be overloaded by downstream modules. In contrast to the
well model where the default class follows a simplistic approach the
default class for aquifiers does nothing, i.e., the actual
implemenentation must be provided by downstream.
