costmap_2d_node: enable setting namespace from launch files

[tonynajjar]

Feature description

I want to run a standalone costmap node and set its (parent) namespace from the launch file because it is dynamically generated.
Currently this is not possible as stated in this comment

This is a feature request to enable this all while retaining the current functionality of setting the namespace via NodeOptions

If you agree, I can contribute here with some guidance what you'd like the design to look like

[SteveMacenski]

Seems sensible. You may be able to find the namespaces / remaps in the main.cpp file that you can just populate into the constructor so that it uses that code to perform what you want. Its a round-a-bout way of getting the same behavior, but is the lowest technical effort I believe. Though, if you can find another way to messing with node options that doesn't break current behavior, have at it!

Edit: actually, another option would be to break this constructor apart and make a new constructor that doesn't call this line, and use that for the node. That's probably the cleanest solution.