Run all tools by default if no “--tools” is given?

The `HOWTO.rst` claims that all celestial benchmarks can be run by specifying `--tools all` at the command line, but the current version of `run_benchmark.py` does not in fact support the special tool name `all`. This pull request restores the run-all functionality, but more simply: instead of detecting a special name, it assumes that all tools should be the default unless a more specific `--tools` option is given.

If this change is too disruptive, then simply reject it and I will instead offer code that makes `--tools all` work as in the current documentation.