Function to calculate trajectory DSG field path length

It would be nice, and should be fairly simple, to implement functionality to calculate the path length of a trajectory DSG field.

Some concrete examples would be: for a DSG field representing a flight trajectory or a storm track - the path length would be the distance travelled on the Earth/sphere by the flight or storm, at least according to the sampling of the trajectory encoded by the data, which could be useful for calculations.

We already have functionality to find distances on the sphere given the sphere's radius, usually taken as the Earth's radius, so that means most of the work required is already done.

I think either a new function cf.path_length or a new method on a field f.path_length (erroring immediately if the field isn't appropriate i.e. not a 2D DSG) would be a good way to enable this, where the name could be this or something similar, as a suggestion.

Start by locating the existing functionality for distances on a sphere and the representation and validation of 2D trajectory DSG fields. Define whether the public entry point is cf.path_length or f.path_length; done should mean calculating the sampled spherical distance for valid trajectories and rejecting inappropriate fields.