catkin_create_pkg boilerplate
Basic changes to boilerplate stuff
Now subscribing to command line provided topics, but have to be all Float32 for now
Next idea is to provide a dict of paramater names like  ./topic_op.py "{'a': '/test1', 'b': '/test2'}" fda
Can now do an arbitrary expression with provided topics
Possibly safer, though not sure.  Now how to get message types so I can use more than Float32s?  Maybe look into what rqt plot is doing with the topic fields.