Set initial latitude / longitude

[chapulina]

We're currently filling the lat/lon coordinates here:

https://github.com/osrf/lrauv/blob/39a08bbcf9acb3e6131c8c38b6d762f0532401ee/lrauv_ignition_plugins/src/TethysCommPlugin.cc#L448-L453

By default, that's considering the world origin to be at 0° / 0°.

We should be setting the correct initial lat/lon at startup so that subsequent queries get the position with respect to that. Here's an upstream issue for setting it from SDF: gazebosim/gz-sim#981

Here, instead of hardcoding it into the SDF, we may be able to subscribe to this message from LRAUV to get the initial values:

https://bitbucket.org/mbari/lrauv-application/src/master/Source/simulatorModule/SimInitStruct.h

[mabelzhang]

Wow, that SimInitStruct is humongous...

[braanan]

Well yeah, it basically holds all the stuff we currently have in the add files..

[mabelzhang]

We could either have a full protobuf message with all those fields or just have a small one that we build up as needed.