Ground electrode should be optional

[asoskic]

Like with reference, users should be first asked whether there was a ground electrode or an alternative system such as the one with DRL was used. We might also want to have this for EOG and other non-EEG electrodes. However, we have already sent the table to Dejan as a final version for V1, and there is an option for people to say "no ground electrode" because it is a text question. This can be something that is improved in Version 2.