Open PR to do dataset slicing

[jerabaul29]

Reminder to myself: open a PR to do dataset slicing.

[knutfrode]

I made now two methods crop and contained_in:
https://opendrift.github.io/trajan/generated/xarray.Dataset.traj.crop.html
https://opendrift.github.io/trajan/generated/xarray.Dataset.traj.contained_in.html

See here for a demonstration of contained_in:
#147

and here for a demonstration of crop:
https://opendrift.github.io/trajan/gallery/example_drifters.html#sphx-glr-gallery-example-drifters-py

[gauteh]

Is this solved?