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 only relevant people who are masochistic enough to go beyond
`-Wall`. (note that at this warning level, there is plenty of noise from
Dune and other upstream dependencies.)
so far, the linker bailed out due to duplicate definitions of
variables if multiple compile units used the same type tag. This is
problematic if the sources are split into separate compile units and
that use the same type tag; in particular, this applies for
traditional libraries.
Due to various C++ peculiarities, this patch complicates the internal
implementation of the property system quite a bit, but given that the
usage of it (as well as the compile time) stay unchanged, I do not
consider this to be a big problem. Note that the introspection code is
particularly problematic because it needs static initializers that do
not cause the linker to choke in the case of multiple compile units.
Finally, to prevent future regressions, this patch adds a unit test
for the lens problem which uses multiple compile units. (This test is
called lens_immiscible_ecfv_ad_mcu and basically identical to the
existing lens_immiscible_ecfv_ad test and I thus think that it is
pretty unimaginative -- improvement proposals are welcome.)
