Support sf

[Robinlovelace]

Currently the package supports sp representations of trajectories but not the newer sf representations. Interested in this and up for contributing in some way to this.

[edzer]

Some initial explorations are found here: https://www.r-spatial.org/r/2017/08/28/nest.html

[Robinlovelace]

trip package supports export to sf: https://github.com/Trackage/trip