Create standardized structured obstacle data message

[mbr4477]

Create a standard message to pass structured obstacle data between the nodes.

[mbr4477]

@irowebbn what's the status of a structured obstacle message? Not sure if we've defined this yet.

[irowebbn]

Right now I have the ProcessLidar Node just publishing an updated heading that avoids obstacles. All the obstacle data stays internal to that node. I realize now this doesn't quite fit the architecture diagram you made. We can split it into two separate nodes if it makes more sense.