Made method to crop dataset to a given region. Added tests and demons…

[knutfrode]

…tration in example_drifter.py

[knutfrode]

Demonstration of contained_in:

from datetime import datetime
import xarray as xr
import trajan as _
import matplotlib.pyplot as plt

ds = xr.open_dataset('dataset_trajectories_to_use.nc')
# A subset of trajectories contained in a box
dc = ds.traj.contained_in(lonmin=-55, lonmax=0, latmin=40, latmax=60)
# Combining the above with a temporal selection (2022 only)
dct = ds.traj.seltime(t0=datetime(2022,1,1), t1=datetime(2023,1,1)
        ).traj.contained_in(lonmin=-55, lonmax=0, latmin=40, latmax=60)

ds.traj.plot(color='k', label='Full dataset')
dc.traj.plot(color='r', label='Subset')
dct.traj.plot(color='g', label='Subset for 2022 only')

plt.show()