This is the more natural situation if some of the norms are actually
estimators. In addition, the EOC should not be affected anyway.

These variable names make reading the error message a bit easier.

Conflicts:
	dune/stuff/test/common_logger.cc

Somehow size_t(...) was 791 while ... was 792, so a std::round(...) might
be better.

Success was previously defined as the results being less or equal to the
expected results, which is not optimal. From now on it is checked whether
the expected result and the result (in scientific notation) are equal in
their first three meaningful digits.

This one should me renamed to main.hh once the deprecation grace period
for the old main.hh is over. The .cxx suffix is there to avoid automatic
test generation for this file.

std::cout is in iostream, not ostream

This reverts commit 1d8919e72d7f91e1034abbbfc6af86432a3f180d.

We can just use `at()`.

This checks the outcome of a convergence study using gtest expectation
macros.