Special relativistic handling of velocities

[attipaci]

• Apply special relativistic corrections in rad_vel() / rad_vel2() properly:
  • using precise γ factor instead of low-velocity approximation
  • using relative motion of source vs observer (rather than the co-added relativistic corrections for source vs barycenter and observer vs barycenter)
  • Applying relative gravitational terms appropriately
• Apply Solar & Earth potentials in rad_vel() / rad_vel2() consistently, both at source and at observer.
• Consistently add velocities using relativistic formula throughout novas.c.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.64%. Comparing base (0a74a51) to head (4ae8676).

Additional details and impacted files

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