Solve compatibility issue by using pandas newer version and matplotlib for plotting

[Fujikawas]

For the newer pandas version(2.2.0 for my instance), it requires the data to be converted to a numpy array before indexing for plotting with matplotlib.pyplot.plot(). Otherwise it gives error like:

ValueError: Multi-dimensional indexing (e.g. `obj[:, None]`) is no longer supported. Convert to a numpy array before indexing instead.

The plot-trajectory.py in the tutorial case oscillator is this case.

[Fujikawas]

Haven't tested if this works for older version yet. Maybe it's better to add a conditional clause here to decide if the conversion is needed.

[BenjaminRodenberg]

No need for backwards compatibility. This is a helper script and I think we can expect the (few) users of this script to have a recent version.