Integrate demo changes into master

[JonasVautherin]

Some lessons have been learned from the local_planner demo. A few values were hard-coded for the sake of the demo, and should be merged into master.

Refactor the demo branch and merge what needs to be kept into master.

NOTE: this issue is meant to address PR #10.

[mrivi]

I don't think it's a good idea to merge the demo branch into master. In the demo branch there is different behavior (if obstacle in front of the drone go back 1m, if obstacle removed move forward 1m) from the normal local planner. In order to achieve it there are a bunch of if statements in the code.

I would propose to keep a separate demo branch to be used if we want to showcase this behavior.

[JonasVautherin]

Agreed.