Suggestions to #148

[chapulina]

• Address codechecker warnings
• Added tests
• Pass and return const &
• Changed the snap API to Position / Rotation / Scale, to better match APIs of other Ignition libraries, like math, rendering, common...
• I think the "preview" concept is very specific to Gazebo and its server-client architecture. Other applications may be interested in using this event to just spawn anything into a scene. I think we'll only really know once we start making use of that event in Scene3D.

[codecov]

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[JShep1]

LGTM, thanks for the tweaks