Add laplace reference method to mne/io/reference.py

Add Laplace reference method to mne/io/reference.py

@verbose
def set_laplace_reference(inst, central_ch, adjacent_chs, copy=True,
verbose=None):
"""Re-reference selected seeg channels using a laplace referencing scheme.

A Laplace reference takes the difference between one central channel with 
the average of adjacent channels. The exception, however, is for the top 
and bottom electrodes in the seeg-shaft, where only one adjacent electrode 
is subtracted (actually, the average of two same adjacent channels).  

Multiple central channels and adjacent channels can be specified.    

Parameters
----------
inst : instance of Raw | Epochs | Evoked
    Data containing the unreferenced channels.
central_ch : str | list of str
    The name(s) of the channel(s) to use as central channel(s) in the 
    laplace reference.
adjacent_chs : list of strs
    The name(s) of the channel(s) to use as adjacent channels in the 
    laplace reference.    
copy : bool
    Whether to operate on a copy of the data (True) or modify it in-place
    (False). Defaults to True.
%(verbose)s

Returns
-------
inst : instance of Raw | Epochs | Evoked
    Data with the specified channels re-referenced.

For example,
-----------
central_ch=['A1','A2','A3']
adjacent_chs = [['A1','A1'],['A1','A3'],['A2','A2']]

See Also
--------
set_eeg_reference : Convenience function for creating an EEG reference.

Reference
---------
Li, G., Jiang, S., Paraskevopoulou, S.E., Wang, M., Xu, Y., Wu, Z., ...
& Schalk,G.(2018).Optimal referencing for stereo-electroencephalographic
(SEEG) recordings. NeuroImage, 183, 327-335.

"""
_check_can_reref(inst)
if not isinstance(central_ch, list):
    central_ch = [central_ch]

if not isinstance(adjacent_chs, list):
    raise ValueError('Adjacent channels are a list of at least two' 
                     'adjacent channels! ')

if len(central_ch) != len(adjacent_chs):
    raise ValueError('Number of central channels (got %d) must equal '
                     'the number of adjacent groups of channels (got %d).' 
                     % (len(central_ch), len(adjacent_chs)))
if copy:
    inst = inst.copy()

# Do laplace reference by multiplying the data(channels x time) with
# a matrix (n_central_channels x channels).
multiplier = np.zeros((len(central_ch), len(inst.ch_names)))
for idx, (a, c) in enumerate(zip(central_ch, adjacent_chs)):
    multiplier[idx, inst.ch_names.index(a)] = 1
    adjacent_ch_n = len(c)
    for i in range(adjacent_ch_n):
        multiplier[idx, inst.ch_names.index(c[i])] += -1/adjacent_ch_n        

# Rereferencing of data.
data = multiplier @ inst._data
inst._data = data

return inst

Start in mne/io/reference.py at the proposed set_laplace_reference entry point and compare it with the existing set_eeg_reference API. Confirm the documented central and adjacent-channel cases, then verify that the returned Raw, Epochs, or Evoked data has the requested Laplace reference applied.