Add LoadMap service

[jacobperron]

The service is for loading a map as an nav_msgs/OccupancyGrid given a resource ID. It also features a result member to encode errors. I tried to make the result codes generic enough for support of arbitrary backends and underlying data structures.

This could be implemented in a map_loader node or directly in map_server seeing how it already "loads" a map on startup.

[jacobperron]

Attempt to resolve #94

[tfoote]

Thanks for submitting this. It's updated the service to use a string id instead of a path. And having the result codes is a good addition. However it needs a very clear semantic of how to resolve the resource id such that alternative implementations would behave similarly.

[jack-oquin]

Making it a string requires clearly defining the semantics. The obvious choice is a URL, with at least these options provided:

• file:///path/to/local/file.yaml
• package://our_maps_package/maps/floor3.yaml

Other nice-to-have schemes could include http:, https:, and remote file:.

[doisyg]

Can we merge this? This is blocking for : ros-planning/navigation#461

[jacobperron]

Since I've lost access to the origin repository, I've opened a continuation PR #152. I've made changes there to switch to URL semantics.