Refactor protobuf messages out to _msgs package

[mabelzhang]

Tasks in this ticket, elaborated below.

• Refactor protobuf messages to lrauv_msgs package
• Decide on style of field names in custom protobuf messages
• Change LRAUVInit to LRAUVSpawn, see https://github.com/osrf/lrauv_private/pull/90#issuecomment-941855914

Refactor to lrauv_msgs

As discussed here #82 (comment) , there is a circular dependency between lrauv-application (MBARI repo) and lrauv_ignition_plugins package.
MBARI repo depends on the compiled protobuf message headers in lrauv_ignition_plugins.
lrauv_ignition_plugins depends on the MBARI repo for automated mission tests.

Verbally, we agreed on refactoring the custom protobuf messages to a lrauv_msgs package.
Then lrauv-application would depend on lrauv_msgs.
lrauv_ignition_plugins would depend on lrauv_msgs and lrauv-application.
That solves the circular dependency.

Style of field names

As discussed here #90 (comment) , we want to eliminate the trailing underscores of the field names, and to change to snake_case.

This applies to LRAUVState, LRAUVCommand, and LRAUVInit.

[chapulina]

Another question about messages, do the non-action fields of LRAUVCommand ever get used? i.e.

https://github.com/osrf/lrauv/blob/9c73845d3d71e60916816a8dedd3a3ba6e7187b4/lrauv_ignition_plugins/proto/lrauv_command.proto#L35-L41

[mabelzhang]

dropWeightState_ is used. That's the only field for drop weight – it doesn't have an action field. It's a state as opposed to an action because on the real vehicle, it's an indicator meaning "it's been dropped. Goner."

The 5 other fields, if I remember correctly, are what the MBARI controller thinks the positions of the actuators are. The Ignition side doesn't use those at the moment. They become useful if there is a mismatch between where the controller thinks the actuators are, and where the simulator's state thinks the actuators are, which can be due to time sync issues, physics instability at a very fast time step, etc.
@braanan can correct me.

[braanan]

All true. Thanks, @mabelzhang