Discovery of eigen3 was brokn for me. Although pkg-config would find the
local eigen3 installation pkg_check_modules would not find it. Since the
FindEigen3.cmake does support hints fora dditional search dirs I hardcoded
the additional location which is currently used by out setup.

Those can always be included!

The other alternative would be to instantiate the Config() in a way that
there is no default warning.

afaict nothing is relying on the implicit type conversions

Since the timed logger is by now instantiated in main.hxx to be used in
other tests this test must not use main.hxx but have its own main.

Drop support for override and final keyword discovery since all supported
compiler understand those by now. Further use of DS_OVERRIDE or DS_FINAL
will trigger a compile error.

see also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47226

storage_access() should be deprecated at some point...

E.g. print 1 instead of [1] for scalar vectors, [1 2] instead of [[1 2]]
for matrices with only one row...

If you did rely on `common/logging.hh` to include `common/timedlogging.hh`
your code may not compile anymore.

is_hashable<T>::value is true iff std::hash<T>(T t) works or gcc is too
old

I see no harm in it atm.