Take online reference channel into account for average reference + add re-referencing for raw/continuous data

[jschepers]

To ensure that the correct number of channels is considered for the average re-referencing (mne-tools/mne-python#13618) I added the following functionality:

• I introduced a function set_initial_average_reference that optionally adds the online reference (specified by the user in the config file) to the data before computing an average reference. The function uses an average reference projection but does not apply it yet.
• The application of the projection happens outside of the function. Afterwards, optionally one can drop the reconstructed online reference channel again. Note: if it's not dropped then there will be one more channel in the data than before.
• I also implemented that the reference specified in the config file is also applied to the raw/continuous data (not only to the epoched data). Thereby this issue fixes Re-referencing for raw (continuous) data only seems to be applied in certain cases #1169.
• After a few discussions we decided to add the re-referencing before the application of the ICA and not do add it again afterwards because currently it's not completely clear to us whether that's desired or even appropriate.

I'm happy to hear what you think about this approach.

Cheers,
Judith

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