Function calceph_compute_unit 

pub unsafe extern "C" fn calceph_compute_unit(
    eph: *mut t_calcephbin,
    JD0: f64,
    time: f64,
    target: c_int,
    center: c_int,
    unit: c_int,
    PV: *mut f64,
) -> c_int

compute the position <x,y,z> and velocity <xdot,ydot,zdot> for a given target and center at a single time. The output is expressed according to unit