Draft: Add InspiralENIGMA

Description

This MR adds the InspiralENIGMA waveform model, which is a time-domain eccentric aligned-spin inspiral-only approximant with higher modes. This is an extension of the eccentric non-spinning inspiral-merger-ringdown approximant published here, which includes BH spin-spin effects in the two-body dynamics, and higher-modes in the gravitational waveforms.

API Changes and Justification

Backwards Compatible Changes

• This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions
• This change adds new classes/functions/structs/types to a public C header file or Python module

Backwards Incompatible Changes

• This change modifies an existing class/function/struct/type definition in a public C header file or Python module
• This change removes an existing class/function/struct/type from a public C header file or Python module

If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.

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There are minimal API changes:

• in the existing XLALSimInspiralChooseTDWaveform method in lalsimulation/lib/LALSimInspiral.{c,h}, that is the gateway to time-domain waveform models, one of the input arguments (meanPerAno) was marked UNUSED. This MR removes that label.
• In the same function, the new approximant introduced is now being called.

Review Status

Review is to be undertaken by the waveform group with the conjunction of other waveform models. The review is yet to organize.