Add GPS

[osrf-migration]

Original report (archived issue) by Brian Gerkey (Bitbucket: Brian Gerkey, GitHub: gerkey).

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• Add gps sensor to model, similar to:

<sensor name="gps" type="gps">
  <gps/>
  <always_on>1</always_on>
 </sensor>

• Add spherical coordinates to each world, similar to (but look up some real coords):

 <spherical_coordinates>
  <latitude_deg>0</latitude_deg>
  <longitude_deg>0</longitude_deg>
  <elevation>0</elevation>
  <heading_deg>0</heading_deg>
</spherical_coordinates>

• Expose this information via new API in RobotPlugin.

[osrf-migration]

Original comment by Brian Gerkey (Bitbucket: Brian Gerkey, GitHub: gerkey).

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• Edited issue description

[osrf-migration]

Original comment by Brian Gerkey (Bitbucket: Brian Gerkey, GitHub: gerkey).

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• changed state from "new" to "resolved"

pull request #8