Use animal ID as default for assigning tracks

[sarahcd]

In most cases, users expect to segment data by animal and see their local individual IDs within the workflow and outputs. The use of deployments is in general most sensible if (1) there are animals with multiple deployments and (2) the deployments represent distinct data segments and were organized in Movebank accordingly, but this is often not the case. In this case the owner should be aware of this and can choose one of the other options.