fixes for compile with gazebo 9

[davefeilseifer]

kinetic-devel version of pr2_simulator is not compatible with gazebo version used with 18.04/melodic due to use of deprecated functions. These commits fix that problem.

[v4hn]

Looks good to me as long as you create a new branch `melodic-devel` for it.

[davefeilseifer]

I'm actually not able to create branches, so I cannot. However, if you're willing to create the branch, I can update the pull request.

[nameofuser1]

@davefeilseifer have you managed to successfully run a simulation using Melodic?

Just today I did the same fixes as yours. However I met other issues when starting simulation. In particular I got error about wrong argument when spawning model. Haven't looked deep into it because there may be plenty of other bugs and I hope that someone have already handled it.

[davefeilseifer]

I haven't checked for runtime errors, just resolved the compile errors. I don't know this package so well, so I'm not likely to dig too deep into this in the near future.

[v4hn]

I'm actually not able to create branches, so I cannot. However, if you're willing to create the branch, I can update the pull request.

@davefeilseifer I just did that and retargeted the request. Thank you for the patch.

@nameofuser1 If you found errors with melodic (I'm rather convinced there are some new ones aside from the API change here), please submit a fix for them!

[k-okada]

thanks, I have cherry-picked this PR to kinetic, with `#if GAZEBO_MAJER_VERSION >=8' switch, so that we still can keep one source tree for working ROS_DISTROs, ( otherwise, we have to cherrypick from one to another every updates)

[k-okada]

@nameofuser1 I think you have encountered -ros_namespace /gazebo error. I think we fixed this by this patch ->
ea35683