This is just an issue of what the default is?  I'm fine either way, since `set_delimiter` makes anything possible, but I think it's nice to build on existing practice of how Boost does it.

Regarding existing practice, my code has been using the default of no commas for almost four years.  I've had users complain about lots of things, but never that.  Here's an example from my code's configuration file
```
initialization
  ambient Pressure    98600
          Velocity    (0 0 0)
          Temperature 300
end
```