Implement a show_isochrons function for section

[amoodie]

Include the ability to overlay isochrons on any axis.

sc8cube.sections['demo'].show('velocity', data='strat', ax=ax[1])
ax[1].plot(sc8cube.x, sc8cube.strata[:,10,:].T, color='k', linewidth=0.5)

This should be a new method of the Section classes. Could be an optional kwarg to show_section and section.show() which calls this new method too.