ISDs with states that contain no velocity (or 0 velocity)

[AustinSanders]

ISIS allows states to be specified with no velocity (Instrument Position, Sun Position) and contains a "hasVelocity" function to test for the presence of velocities. However, ALE's formatter does not allow for writing ISDs that contain states without velocities. In order to maintain parity with ISIS and support additional ALE drivers (like ApolloPanIsisLabelIsisSpice), what is the best way to support ISDs that have states with no velocity?

I created a PR that uses np.zeros to mimic the shape of the positions, but I'm not sure if this will have side effects. Creating an empty list ( velocity = [] ) in the isd seems like a more intuitive solution, but deserializing the empty JSON list is causing issues.

[AustinSanders]

Based on comments on #614, removing the 'velocities' key altogether if no velocities are present is the correct way of handling this.