Allow baseline time window selection from other events

Describe the new feature or enhancement

Currently, all of the baseline inputs (for example in the Epochs class) require a tuple of start and end times. This assumes you want to calculate your baseline with respect to the same event which you are epoching from.

Lets say I have two events: 'e1' and 'e2'. I want to create an epoch that goes from -1s to 2 seconds around 'e2' with a baseline correction from -0.5s to 0 with respect to 'e1'

events, event_id = mne.events_from_annotations(raw)
e2 = mne.Epochs(raw, events, event_id['e2'], tmin=-1, tmax=1.5, baseline=None)
baseline = mne.Epochs(raw, events, event_id['e1'], tmin=-0.5, tmax=0, baseline=None)

e2 is the time frame I'm interested in, and baseline is the data I'd like to use for baseline correction. Setting baseline=(-0.5,0) would only do so with respect to 'e2'.

Describe your proposed implementation

e2 = mne.Epochs(raw, events, event_id['e2'], tmin=-1, tmax=1.5, baseline={event_id['e1'] : (-0.5,0)})

Describe possible alternatives

e2 = mne.Epochs(raw, events, event_id['e2'], tmin=-1, tmax=1.5, baseline=(-0.5,0), baseline_event=event_id['e1'])
e2 = mne.Epochs(raw, events, event_id['e2'], tmin=-1, tmax=1.5, baseline=(event_id['e1'],-0.5,0))

Additional context

No response

Start with the mne.Epochs entry point and trace how its baseline argument is interpreted relative to selected events. Define the behavior for using e1 as the baseline while epoching around e2, including how the proposed input form should be validated and documented; the issue is complete when that workflow works reliably.