Set specifications for eye-tracking file name handling

[jschepers]

Changes:

• Before the eye-tracking files needed to have task and run specified. I have now introduced two variables in the config file et_has_run and et_has_task, that indicate whether the eye-tracking file names have task and run specified or not.

  If they are set to False, it is assumed that the file name does not have task or run. If they are set to True , task and run are taken from the EEG data.

• I commented out the _update_for_splits function calls for the eye-tracking data paths. We still need to find out whether we need them or not.

Before merging …

• Changelog has been updated (docs/source/changes.md)

[jschepers]

I tested this functionality once for "False" once for "True" and the resulting file paths looked reasonable to me but I did not write any tests.

[behinger]

Looks good to me!

[jschepers]

Thanks for having a look! Then I'll merge.