[melodic] Gazebo Dependency and GetModel Fixes

Stretch Gazebo for ROS1

Rospack fails to find Stretch packages unless setup is sourced

bug fix for obstacle detection during FUNMAP nav

bug fix for tilt backlash

The tilt backlash model is supposed to compare the current tilt
angle of the head with the head_tilt_backlash_transition_angle
to determine if head_tilt_looking_up element of the backlash state
is true or false. This is due to the head resting against
different sides of the tilt joint based on whether it's looking up
or looking down.

At some point, an error was introduced in which
head_tilt_looking_up was set based on the recent relative motion
of the head tilt joint. This is the way backlash state is
determined for the telescoping arm and head panning.
In contrast, the head tilt backlash state depends on gravity
and where the center of mass of the head is relative to the
tilt joint's axis of rotation. As such, the head tilt backlash
state depends on the tilt angle of the head.

We are not currently focusing our development on stretch_demos and stretch_funmap.

Add missing dependency on realsense (for devel branch)