Support joint force commands with the Battery plugin

[nkoenig]

Joint force commands were not checked in the battery plugin.

To test:

1. ign gazebo -v 4 examples/worlds/lift_drag_battery.sdf
2. Listen to the battery topic: ign topic -et /model/lift_drag_demo_model/battery/linear_battery/state
3. Run simulation.
4. The battery should be full and not discharging.
5. Start the propeller: ign topic -t "/model/lift_drag_demo_model/joint/rod_1_joint/cmd_force" -m ignition.msgs.Double -p "data: 0.7"
6. The battery should start discharging.

Signed-off-by: Nate Koenig [email protected]

[codecov]

Codecov Report

Merging #165 into ign-gazebo2 will increase coverage by 0.09%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           ign-gazebo2     #165      +/-   ##
===============================================
+ Coverage        62.25%   62.35%   +0.09%     
===============================================
  Files              123      123              
  Lines             6101     6101              
===============================================
+ Hits              3798     3804       +6     
+ Misses            2303     2297       -6     

Impacted Files	Coverage Δ
src/SimulationRunner.cc	88.77% <0.00%> (+1.20%)	⬆️

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[caguero]

Just one comment, other than that ready to approve.