OARIS — No Unknown / No Statement value for environment

The sensor_track_type class does not allow a track to not have a defined environment value. Whilst the environment may be known or assumed for most cases of a naval radar track, for other types of sensor this is not necessarily the case.

Environment should be optional in sensor_track_type

Make attribute environment optional in sensor_track_type - environment_type then continues to enumerate actual environments and so it is possible to use this type in a non-optional (mandatory) context to define an object as being in a definite environment.