Add more model APIs

[chapulina]

Partially addresses #325

Added a new tutorial / guide which lists all of gazebo::physics::Model functions and how they map to existing Ignition Gazebo functionality.

Also added several functions to Model.hh and Util.hh.

Finally, I made more of the model's components be displayed on the component inspector, related to #158.

[codecov]

Codecov Report

Merging #349 into ign-gazebo3 will increase coverage by 0.00%.
The diff coverage is 81.73%.

@@             Coverage Diff              @@
##           ign-gazebo3     #349   +/-   ##
============================================
  Coverage        77.32%   77.32%           
============================================
  Files              197      197           
  Lines            10530    10577   +47     
============================================
+ Hits              8142     8179   +37     
- Misses            2388     2398   +10     

Impacted Files	Coverage Δ
include/ignition/gazebo/Model.hh	100.00% <ø> (ø)
...e/ignition/gazebo/components/AngularVelocityCmd.hh	100.00% <ø> (ø)
...de/ignition/gazebo/components/LinearVelocityCmd.hh	100.00% <ø> (ø)
.../plugins/component_inspector/ComponentInspector.cc	9.09% <0.00%> (-0.24%)	⬇️
src/systems/physics/Physics.cc	70.19% <0.00%> (-0.93%)	⬇️
src/Model.cc	95.89% <93.18%> (-4.11%)	⬇️
src/Util.cc	94.80% <100.00%> (+1.58%)	⬆️
src/ign.cc	72.22% <0.00%> (-0.76%)	⬇️
src/systems/log/LogRecord.cc	81.90% <0.00%> (-0.46%)	⬇️
src/systems/log/LogPlayback.cc	60.95% <0.00%> (-0.19%)	⬇️
... and 5 more

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[JShep1]

LGTM, I left a couple small nits and a question, but pending that and CI, this should be good to go.

[JShep1]

Pending CI this is good to go!