New example field: discrete sampling geometry trajectory features

[davidhassell]

It would be useful to have a "trajectory" DSG example field that can be return by cfdm.example_fields. E.g.

>>> cfdm.example_field(11)
Field: mole_fraction_of_ozone_in_air (ncvar%O3)
-----------------------------------------------
Data            : mole_fraction_of_ozone_in_air(cf_role=trajectory_id(1), ncdim%trajectory(4)) ppb
Auxiliary coords: time(cf_role=trajectory_id(1), ncdim%trajectory(4)) = [[2024-02-26 09:01:00, ..., 2024-02-26 09:04:00]] standard
                : altitude(cf_role=trajectory_id(1), ncdim%trajectory(4)) = [[2577.0, ..., 2563.0]] m
                : air_pressure(cf_role=trajectory_id(1), ncdim%trajectory(4)) = [[751.0, ..., 780.0]] hPa
                : latitude(cf_role=trajectory_id(1), ncdim%trajectory(4)) = [[52.0, ..., 52.2]] degree_north
                : longitude(cf_role=trajectory_id(1), ncdim%trajectory(4)) = [[0.0, ..., 0.31]] degree_east
                : cf_role=trajectory_id(cf_role=trajectory_id(1)) = [flight1]