Improved mecanum wheel model

[VladimirIvan]

• Changed to use a passive rolling ball as a model instead of modifying the friction
• Removed the following parameters: fixedLinkName, rollerAngle, rollerFriction, rollerSkidFriction
• Added axis parameter
• Removed dependency on ODE
• Updated for compatibility with Gazebo 9
• Added readme
• Updated changelog
• Did not change version number
• Only supported for ODE and Dart

[civerachb-cpr]

If you're still working on this PR can you please mark it as WIP?
If it's ready to be reviewed, can you please rebase and address the merge conflicts above; it should not be necessary to manually edit the Changelog; that file gets automatically updated when we release the package.

[VladimirIvan]

Hi @civerachb-cpr,
it looked like the repo is not supported any more so I the PR slipped my radar. It is now rebased and finished. Please go ahead with the code review.

[civerachb-cpr]

Please revert the change to the package maintainer.
Everything else looks generally OK, but I'll need to run some tests before I can approve the PR.