Doc for GriddedLatLonSet and a few Trajectory improvements.

Remove Trajectories with less than one complete pair before building depiction geometry. This may happen if a Trajectory is initialized close to a grid edge and
the first displacement carries it off the grid.

Enhance the documentation a bit.

return the scan local coords

Trajectory: make sure we get streamlines with careful selection of control parameters.

Trajectory enhancements...

initial time at which a trajectory will be created.

Runge-Kutta (RK4) determination of parcel trajectory path