Convenience methods for Horizons objects

[moble]

The Horizons object currently has a newtonian_com property and an average_com_motion method. It might be nice to include several other features needed for the LVC format, which they name

• nhat: the separation vector from BH B to BH A (x_A - x_B)
• omega_orbit: the orbital angular frequency
• LNhat: the direction of the (Newtonian) orbital angular velocity

• newtonian_com
• average_com_motion
• n⃗
• n̂
• λ̂
• ℓ̂
• chi_prec (Sec. 3 of https://arxiv.org/abs/1408.1810)
• Sigma and Sigma__perp_λ̂ (Eq. (5) of https://arxiv.org/abs/1409.4431)