Fix: make __getitem__ delegate to states array

[ftavella]

Summary

Fixes #19

Replaces the old __getitem__ that returned (time, state) tuples with direct indexing into trajectory.states. The new implementation supports the full numpy indexing syntax:

trajectory[0]          # all states at time index 0
trajectory[0, 1]       # state 1 at time index 0
trajectory[0, 1, 2]    # state 1, batch 2 at time index 0
trajectory[:10]        # first 10 time points, all states

Changes

• __getitem__ on DynamicalTrajectory now delegates directly to self.states[idx]

Tests

Updated test_models.ipynb to replace old tuple-based assertions with tests covering single indexing, multi-dimensional indexing, slicing, and batch indexing.