Add optional parameter to generate Fixed/Coninuous caster joint

[BarceloChristian]

🎉 New feature

Closes #99

Summary

To improve simulations, the caster wheel must rotate freely when the robot moves
By default, the joint keeps fixed as of now. Alternatively, the option use_fixed_caster:=False can be used when generating the URDF from XACRO.

Test it

• Build the repository (needed to update the imported xacro)
• Move to andino.gazebo.xacro location (can be done with base description too)
• Run:

xacro andino.gazebo.xacro > andino.xacro use_gazebo_ros_control:=False use_fixed_caster:=False

• Check the caster structure is connected to the base link and the wheel using continuous joints.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if it affects the public API)

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[francocipollone]

Related to #79

[francocipollone]

LGTM

Thanks for the contribution.

I think that this fix is quite simple and doesn't modify the default robot behavior.
Btw, in the future the swivel caster will be changed most probably by a ball caster.