Support vertical (not just horizontal) trajectory plots via `cfp.traj`

It would be useful to allow users to create a plot showing the vertical ('Z') evolution of a trajectory with respect to another coordinate, notably time - for example, with a flight trajectory, you would expect to see the flight's vertical coordinate, if as an altitude, increase from ~0 (m) as it takes off (or if pressure is the Z coordinate, decrease from ~1000 hPa) and a plot would show exactly how the flight ascended in height from ground level over time, or per some reduction or cross-section in horizontal space etc.

Given traj is our method to plot trajectories, currently only in horizontal form, I think it makes most sense to provide such support by generalising that method to allow inclusion of vertical axes. Proposed API for this TBC, but I am initially thinking the best way to go will be via a new keyword argument or two to allow the user to state they want a vertical plot from the provided field. We should also think about the interface of lineplot as a method though - since the ultimate plot will be a line plot.