Add per-group ref_channel_ids to common_reference

[grahamfindlay]

With reference="global" and groups passed (i.e. not None), common_reference() referenced each group to its own channels and ignored ref_channel_ids. I'm not sure if this was intended behavior or a bug. I think a bug, because the docstring stated "a list of channels to be applied to each group is expected", and I read that as "a list of channels per group to be applied to that group is expected". That would be useful for tetrode recordings, because the most principled reference for a tetrode may sometimes be the average/median of all OTHER tetrodes (i.e., don't subtract the tetrode's average/median from itself -- especially with small numbers of tetrodes, this could be dangerous).

Therefore this PR has 2 commits. The second just introduces some convenient syntactic sugar for the functionality provided by the first.

1. The first commit allows ref_channel_ids to be a list of per-group channel-id lists: the reference subtracted from each group is the operator (median/average) over that group's reference set, which may include channels OUTSIDE the group. This enables cross-group referencing (e.g. each tetrode referenced to the median of all channels on the other tetrodes). ref_channel_ids=None (default) keeps the previous own-group behavior.
2. The second commit adds common_reference(..., ref_channel_ids="complement") as syntactic
   sugar for referencing each group to all channels NOT in it -- it just auto-builds the correctref_channel_ids to accomplish this.

So now if you have groups that represent the tetrodes, you can do tetrode-aware referencing with:

import numpy as np
import spikeinterface.preprocessing as spre

groups = np.asarray(rec.get_property("group"))
cids = np.asarray(rec.get_channel_ids())
tetrodes = [list(cids[groups == g]) for g in sorted(set(groups.tolist()))]

# No sugar
complements = [list(cids[groups != g]) for g in sorted(set(groups.tolist()))]
rec_ct = spre.common_reference(
    rec, reference="global", operator="median",
    groups=tetrodes, ref_channel_ids=complements,
)

or

# With sugar
rec_ct = spre.common_reference(
    rec, reference="global", operator="median",
    groups=tetrodes, ref_channel_ids="complement",
)