Add a grouping argument to the movegroup function

[JackAbrand]

I'm interested in aggregating individual utilization distributions (UDs) within multiple treatment groups into a single group-level UD for each of those treatment groups. It would be great to add a 'group' argument to the movegroup function to assign individuals to different groups (e.g. different species, or treatment groups in the data) in order to calculate specific group-level UD's.

[SimonDedman]

See movegroup_extra folder in dropbox, Vital has started the code but it's killing installs so I've moved it for now