Fix: Use data.table's env argument

With #70 using the env argument, refactor all functions to use this safer+cleaner+more readable interface for programming on data.table. 

Ref: https://rdatatable.gitlab.io/data.table/articles/datatable-programming.html

Non-exhaustive list:

- [ ] `centroid_group`
- [ ] `direction_group`
- [ ] `direction_to_centroid`
- [ ] `edge_dist`
- [ ] `fusion_id`
- [ ] `group_pts`
