The preserveFixedJoint option is meant to disable the lumping
of fixed joints by "preserving" the type of a fixed joint,
instead of converting it to a revolute joint with max and min positions limits
set to 0 .
This PR is fully backcompatible: the old option disableFixedJointLumping is still working fine, but if on the same joint both the new option preserveFixedJoint and the old option disableFixedJointLumping are present, then the joint will be converted to a fixed joint. This is meant to simplify the life of model creators that need to support both the old version of Gazebo (and hence sdformat) without support for preserveFixedJoint and the new version that can exploit this new option.