Add simple_office simulation map

[rchiossi]

simple_office is a more realistic simulation scenario for testbed.

[mbelluzzo]

Although Gazebo find the models on both places, I do prefer to keep 'models' folder inside 'worlds' instead of 'testbed'. It makes more sense to me since those files are related.

Also, remember to add the signed-off on the commit msg.

Besides that, it looks good!

[rchiossi]

changed and merged.