Kerr equatorial eccentric - prograde + retrograde waveform, better mode handling

[CChapmanbird]

Waveform generator for KerrEqEcc: KerrEccentricEquatorialFlux for both prograde and retrograde waveforms.

• time and frequency domain summation
• online mode selection
• array indexing of modes (as opposed to tuples) for faster waveform evaluations