Battery updates

[caguero]

This patch introduces the following battery updates:

• The recharging is no longer triggered by the parameter socThreshold. The old behavior implies that a battery automatically starts charging when the battery capacity falls below a certain threshold. I think it's more realistic that the recharging can be started or stopped on demand. I took the liberty of removing the socThreshold parameter directly because it wasn't documented in the header file.

• If <enable_recharging> is present, two new services are available for starting or stopping the battery recharge.

• If there's no joint velocity/force command, the battery doesn't drain anymore. Before, once the battery started draining, it never stopped even if the joints weren't moving.

• I updated the battery status published. It can now be in CHARGING, DISCHARGING, FULL or NOT_CHARGING state.

• I documented the SDF charging parameters and added a few minor tweaks.

[codecov]

Codecov Report

Merging #183 into ign-gazebo2 will not change coverage.
The diff coverage is n/a.

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  Coverage        62.35%   62.35%           
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  Files              123      123           
  Lines             6101     6101           
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  Hits              3804     3804           
  Misses            2297     2297           

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[claireyywang]

Thanks for improving the battery feature! I wasn't sure how to test the start/stop recharge function. Could we add some documentation on what service to send in the sdf file?

[caguero]

Thanks for improving the battery feature! I wasn't sure how to test the start/stop recharge function. Could we add some documentation on what service to send in the sdf file?

It's a bit harder to test because we're using a one way service, as we don't expect any response. One way services are not supported in ign command line tool for now. You can create a publisher similar to the one in https://github.com/ignitionrobotics/ign-transport/blob/master/example/requester_oneway.cc and publish it to /vehicle_blue/linear_battery/recharge/start or /vehicle_blue/linear_battery/recharge/stop.

How about I update ign tools to support one way services, and then, I update this example in a separate PR?

[claireyywang]

How about I update ign tools to support one way services, and then, I update this example in a separate PR?

If that's not too much work then I think that'd be very helpful for demo purposes. And yes, separate PR sounds like a good plan.

[claireyywang]

LGTM, just waiting for pending CI to approve

[claireyywang]

Approve since the homebrew test failure doesn't seem to be introduced by this PR. @chapulina I saw a similar test failure in this CI https://build.osrfoundation.org/job/ignition_gazebo-ci-pr_any-homebrew-amd64/3142/