Remnant surrogate models NRSur7dq4Remnant and NRSur3dq8Remnant

Description

Adding remnant surrogate models NRSur7dq4Remnant and NRSur3dq8Remnant, which predict the mass, spin and recoil velocity of the remnant black hole from BBH mergers.

NRSur7dq4Remnant is trained on generically precessing BBH with q<=4 and spin magnitudes |chi_1, chi_2| <= 0.8. This is described in https://arxiv.org/abs/1905.09300.

NRSur3dq8Remnant is trained on nonprecessing BBH with q<=8 and spin magnitudes |chi_1, chi_2| <= 0.8. This is described in https://arxiv.org/abs/1809.09125 but is referred to as surfinBH3dq8 in the paper.

Also add new python interface for calling NR fits, generalizing the previous interface to generically precessing systems.

API Changes and Justification

Backwards Compatible Changes

• This change introduces no API changes
• This change adds new API calls

Backwards Incompatible Changes

• This change modifies an existing API
• This change removes an existing API

Review Status

Review in progress: https://git.ligo.org/waveforms/reviews/RemnantFits/wikis/home