Benchmark gravitation

See 0dbc299fdc4 for the cost of templatisation (between 15% and 20%).
Both before and after, the difference between native and CLR is about 10%.
Copy-pasta from HORIZONS,
>```
>Coordinate system description:
>
>  Earth Mean Equator and Equinox of Reference Epoch
>
>    Reference epoch: J2000.0
>    xy-plane: plane of the Earth's mean equator at the reference epoch
>    x-axis  : out along ascending node of instantaneous plane of the Earth's
>              orbit and the Earth's mean equator at the reference epoch
>    z-axis  : along the Earth mean north pole at the reference epoch
>```
apparently they has no grammars. Fixing.