Fix handling of .edf and .asc eye-tracking files

[jschepers]

Changes

• Problem: In the previous version, the get_input_fnames_sync_eyelink function threw an error if the ET .asc file did not exist, because it checks whether all files with the given paths exist.
• Solution:
  • I implemented a check which of the two files (.asc and/or .edf) exists, already in the get_input_fnames_sync_eyelink and only saved the existing file in in_files.
  • I adapted the sync_eyelink function such that it checks the file extension of the given ET files and either loads the file (if .asc) or first calls edf2asc (if .edf).

Before merging …

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

[jschepers]

I quickly tested the cases:

• neither edf nor asc
• only edf
• asc and edf

But I did not do extensive testing.

[behinger]

I didn't find an obvious problem. I think you should merge.

[jschepers]

Thank you. I solved a small merge conflict and will merge now.