Revise argument names in the `specular.get_specular_trajectory` function to make explicit the expected frame

[vinay0000]

In below function revise the name of the arguments to indicate that the input trajectories should be in the itrf frame (and not the icrf_ec frame). Also revise to indicate that it is the trajectories which are required as input.

def get_specular_trajectory(
    transmitter: StateSeries,
    receiver: StateSeries,
    times: AbsoluteDateArray,
    surface: SurfaceType = SurfaceType.WGS84,
) -> PositionSeries:

Consider doing the same at other similar cases.