Align output formats and data return options between inspiral-only and IMR generation functions

[Copilot]

IMR waveform functions had two inconsistencies with their inspiral counterparts: modes/polarizations were returned first (inspiral returns them last), and there was no return_pycbc_timeseries flag to skip the overhead of packaging numpy arrays into PyCBC TimeSeries objects.

Changes

get_imr_esigma_modes / get_imr_esigma_waveform (generator.py)

• Added return_pycbc_timeseries=True parameter; when False, returns raw numpy arrays with a time grid as the first element
• Fixed return order — modes/polarizations are now last: (orbital_vars_dict, hyb_info, modes_imr) instead of (modes_imr, orbital_vars_dict, hyb_info)
• get_imr_esigma_waveform now calls modes internally with return_pycbc_timeseries=False and handles PyCBC wrapping itself, consistent with inspiral waveform
• Renamed internal inspiral retval → inspiral_retval to disambiguate from hybridization retval

get_imr_esigmasur_mode / get_imr_esigmasur_waveform (surrogate/generator.py)

• Same return_pycbc_timeseries=True addition with numpy+time-grid path
• Fixed a pre-existing bug in get_imr_esigmasur_waveform where unpacking from get_imr_esigmasur_mode incorrectly assumed modes-first ordering (that function already returned modes last)
• Polarizations now returned last in get_imr_esigmasur_waveform

Misc

• Replaced exec() f-string pattern for orbital vars PyCBC wrapping with direct dict assignment

Usage after this change

# NumPy path (no PyCBC overhead):
t, modes = get_imr_esigma_modes(..., return_pycbc_timeseries=False)

# Extra return values now come before modes/polarizations:
t, orbital_vars, hp, hc = get_imr_esigma_waveform(
    ..., return_orbital_params=True, return_pycbc_timeseries=False
)

# Default (PyCBC) behaviour is unchanged:
hp, hc = get_imr_esigma_waveform(...)  # still returns PyCBC TimeSeries

[Akash-Maurya-0899]

@Akash-Maurya-0899 / @copilot Check if this matches with the time-grid generated by the sample_times method of the corresponding PyCBC TimeSeries modes.

[prayush]

@copilot Here and everywhere in this PR, keep the order of return objects such that modes_imr is first and all the others are packed into a tuple. e.g. these lines can be replaced with

        if return_hybridization_info and return_orbital_params_user:
            return modes_imr, (retval, orbital_vars_dict)
        elif return_orbital_params_user:
            return modes_imr, (orbital_vars_dict,)
        elif return_hybridization_info:
            return modes_imr, (retval,)
        return modes_imr