added param, has_param and launch_file substitution arg

added `param`, `has_param` and `launch_file` substitution arg

`param` can substitute ros params
`has_param` can be substituted with bool value whether a ros param exists (e.g. for use in `if` or `unless`)
`launch_file` can be substituted with the name of (one of) the launch file(s) that is parsed until this point, without a given index of which launch file is wanted, the last one is used

also fixed an issue in `_launch_tag` where a ros_config instance where given to the ifunless decorator instead of a context

additionally it is now possible to access `ros_config` for more sophisticated substitution args