Use ROS parameters in transform #21

Description

It would be immensly helpful to be able to set the parameters of transform via a yaml parameter file. For example, in a launch file:

Node(
        package='topic_tools',
        executable='transform_node.py',
        name='transform',
        output='screen',
        parameters=[
            {'input': '/conemap'},
            {'output-topic': '/conemap_rviz'},
            {'output-type': 'visualization_msgs/MarkerArray'},
            {'expression': 'conemap_to_markerarray.convert(m)'},
            {'import': ['conemap_to_markerarray']}
        ])

This way one can make a pip-installable python package, and withou worrying about any ROS implementation details just run it as a very simple ros node. I have a working implementation of this feature in my fork, although I'm a newbie to ros and it's probably full of bugs etc

Completion Criteria

You can use both command line arguments and ros parameters to alter transform settings.