Tolerance tests fail without -std=f2008

[skosukhin]

CI fails if we build the project with gfortran and without the -std=f2008 flag.

We have several options:

1. Update CMake scripts so that the project is always built with the flag if the compiler is gfortran. I don't like this idea because it is not clear what we should do with other compilers. Inject similar flags for them too?
2. Figure out why exactly the flag changes the results and rewrite the relevant part of the code so that it is not sensitive to the flag anymore.
3. Adjust the tolerance threshold. It also makes sense to generate the reference data consistently (preferably with a CI job) with gfortran and no extra flags (i.e. with -DCMAKE_BUILD_TYPE=None -DCMAKE_Fortran_FLAGS=).