KE and PE labels in zundel_I-PI.ipynb have been swapped

[A-614]

Plotting zundel.out:

axs[0].plot(time, (KE - KE[0]), linewidth = 1.5, label = r'$\Delta PE$')
axs[0].plot(time, (PE - PE[0]), linewidth = 1.5, label = r'$\Delta KE$')