Allow Spectrum objects to support complex coefficients

[tsbinns]

Describe the new feature or enhancement

Discussed briefly with @larsoner.

We are considering changes in MNE-Connectivity where the spectral coefficients can be passed directly to the connectivity estimation functions in the form of EpochsSpectrum and EpochsTFR objects, as returned from Epochs.compute_psd/tfr(output="complex").

While the complex output is supported for TFR objects, Spectrum objects do not allow complex outputs from the various PSD functions:

mne-python/mne/time_frequency/spectrum.py

Lines 319 to 323 in 6ad28a4

	if method_kw.get("output", "") == "complex":
	raise ValueError(
	f"Complex output is not supported in {type(self).__name__} objects. "
	f"Please use mne.time_frequency.{psd_funcs[method].__name__}() instead."
	)

Describe your proposed implementation

Remove the ValueError for output="complex" from Spectrum classes.

There are a couple places where tweaks would be required, e.g. in _check_values() and handling PSD func outputs in _compute_spectra(), but similar solutions are already present in the TFR classes.

Same situation for the plotting methods, but again TFR classes also have solutions for this already which could be followed:

mne-python/mne/time_frequency/tfr.py

Lines 4326 to 4328 in 0c369e0

	# complex amplitude → real power; real-valued data is already power (or ITC)
	if np.iscomplexobj(data):
	data = (data * data.conj()).real

One thing that would need to be added is storage for weights in the Spectrum class when complex output is requested from the multitaper mode, which as I understand is not the case for the TFR multitaper.

Describe possible alternatives

N/A

Additional context

No response

[tsbinns]

Addressed in #12747