ENH: Support concatenate_epochs() for EpochsTFR

[aman-coder03]

Reference issue (if any)

towards #13191

What does this implement/fix?

concatenate_epochs() now works with EpochsTFR objects. Before this, passing a list of EpochsTFR instances would just crash with a TypeError because EpochsTFR isn't a subclass of BaseEpochs

fix adds a helper function _concatenate_epochs_tfr() in mne/epochs.pythat takes care of stacking the data, merging event IDs, offsetting event timestamps, and handling metadata, basically the same things the existing concatenation does for regular epochs
concatenate_epochs() now checks if it's dealing with EpochsTFR objects and routes to this helper automatically
tests are added in mne/time_frequency/tests/test_tfr.py

Additional information

task 1 of 3 from #13191 (as discussed with @drammock)
tasks 2 and 3(EpochsSpectrum support and allowing EpochsTFR to accept RawTFR as input) will come in follow up PRs

[drammock]

Suggested change

	shift = np.int64((10 + ref.times[-1]) * ref.info["sfreq"])
	events_offset = int(np.max(epochs_list[0].events[:, 0])) + shift
	shift = len(ref.times)
	events_offset = ref.events[-1, 0] + shift

[nordme]

Hi @aman-coder03 ! Thanks for the PR.

I took a look at the concatenation function and tests, and it's looking pretty good so nice work. I spotted a few more EpochsTFR attributes that need handling.

Attributes to check:

1. ep.baseline
   Even if the times arrays are equivalent across epochs instances, some instances could have baselines where others have none, and that's a problem. For concatenation, baselines should be the same across instances.
2. ep.method
   We should check that the method of construction is the same for all our epochs instances (e.g. not mixing multitaper and morlet wavelet TFR results for example).
3. ep.weights
   This attribute stores weights for tapers. If applicable, these need to be equivalent across epochs instances.

Attributes that need concatenation:

1. ep.comment
   This attribute typically stores trial descriptions of some sort and might not be the same across epochs instances (e.g. 'run 1', 'run2', 'run 3'). These need to be put together for the returned epochs instance.

Cheers.