'Set covariance to be processed as being diagonal.
Returns
cov : dict
The covariance.
Notes
This function allows creation of inverse operators
equivalent to using the old "--diagnoise" mne option.'

def as_diag(self):

    if self['diag']:
        return self
    self['diag'] = True
    self['data'] = np.diag(self['data'])
    self['eig'] = None
    self['eigvec'] = None
    return self
