Tests: Loosen Tolerances Following Mac OS Testing

Testing

After building in both serial and parallel, and running the whole test suite, I found tolerance failures in several tests, which one can attribute to numerical noise. Accordingly, the corresponding tolerances have been loosened. In the context of the Maxwell tests, many tolerances were e-20 or smaller. I've rounded all exponents up to e-18, because I don't see any need to test effectively zero values to (less than?) machine precision. This should make the tests more robust.

I'm tagging the relevant developers, such that if anyone has a reason against a change, they can feed back: @martin.lueders @ftroisi @heiko.appel @nicolastd @micael.oliveira

@martin.lueders and @ftroisi have been added as reviewers, as this follows on from their thorough testing of package handling and building on Mac, which they did an awesome job with!

More justification for each change is given in the commit messages.

Package and Compilation Details

Built on Mac OS 12.6.2 using GNU Fortran (Homebrew GCC 13.2.0) 13.2.0 and formulae:

brew install fftw openblas metis open-mpi gsl

with the cmake build command:

cmake --preset mac-brew-gcc13 --fresh -DOCTOPUS_MPI=On DOCTOPUS_OpenMP=On -DFETCHCONTENT_SOURCE_DIR_SPGLIB=third_party/Spglib -DCMAKE_DISABLE_FIND_PACKAGE_Libxc=On