adopt code to support SIESTA usecase 2.3

[robertoostenveld]

This is needed to deal with the https://github.com/robertoostenveld/Wakeman-and-Henson-2015 pipeline to be used under https://github.com/SIESTA-eu/wp15 on the 1.0.6 version of the dataset https://doi.org/10.18112/openneuro.ds000117.v1.0.6 in BIDS format

[github-actions]

You should test whether your modifications do not break anything.
See https://www.fieldtriptoolbox.org/development/testing/

When outside the DCCN, please consider testing: test_bug46, test_issue1866, test_bug1600, test_bug2664, test_bug2639, test_pull1877, test_bug1775, test_issue1068, test_bug2460, test_ft_componentanalysis_methods, test_bug1405, test_ft_checkdata, test_bug3275, test_issue1459, test_bug2148, test_bug2783, test_bug1014, test_bug2965, test_bug2556

When inside the DCCN, please also consider testing: test_bug2539, test_bug3144, test_bug1315, test_eeglab_ft_integration, test_bug811, test_bug2613, test_bug2559, test_bug1887

Suggested tests outside the DCCN use public data or do not use data.
Suggested tests inside the DCCN use private data.

[schoffelen]

Running the siesta code using this PR's branch works fine. merging...