Merge fatalerror subdirectory into utility
Currently, the error handling utilities in src/gromacs/fatalerror/ use string formatting code from src/gromacs/utility/format.h. It's only a matter of time before code in src/gromacs/utility/ will require some error handling support, and that will then introduce a circular dependency between these two directories, which is not desirable. The simplest solution would be to merge the two, as both error handling and general utility functions feel like they should be on the lowest level in the code.
This will be relatively straightforward to do, but there is a non-zero amount of work involved to keep the code well-organized and to update all relevant Doxygen comments, so it would be better to raise any concerns before the work is started.