Add optional argument to append_output_file

This allows the creation of files that don't have the same root as the run name. The argument is optional, so the default behaviour of using the run name is preserved everywhere.

The motivation for adding this is that I’m merging the init and timestep benchmarks into a single executable*, so need a single executable to produce two different timing files. These are currently called <run_name>.timing.<system_info> files, and I need to replace <run_name> with <descriptive_name>.

*The motivation for this is that the advance time benchmark has to do a full initialization anyway, so it’s silly not to time it (and instead time it in a separate benchmark), particularly when the initialization can be quite expensive.