Support for EyeLink DataViewer protocol

[smathot]

This issue mostly just something to think about.

Quite some people are using the EyeLink DataViewer for analysis. DataViewer uses a simple protocol that requires certain messages. For example, the start of a trial should be:

TRIALID [trialid]

Whereas the OpenSesame plugins use by default:

start_trial [trialid]

And variables are logged as:

!V TRIAL_VAR [name] [value]

Whereas PyGaze uses by default:

var [name] [value]

People can override our defaults, or they can override the DataViewer's defaults. But many people don't realize that, so it's not very user friendly. I'm wondering what the best way to deal with this is.

• We could simply take over the DataViewer protocol. That would make interoperability easiest. The downside of this is that it would break backwards-compatibility. I also don't think their syntax is very readable (what's up with the !V?), but that's maybe not super-important.
• We could allow users to specify whether they want to write PyGaze-style or DataViewer-style messages. If we do that, at what level should this be specified? In the PyGaze Python library, or the OpenSesame plugins?

Thoughts?

[esdalmaijer]

Maybe automatically add an EyeLink-specific log message in the libeyelink implementation? This would mean two messages are logged at every call to the log method. Trial starts and ends we can add in the start/stop_recording methods.

These should then be ignored in PyGaze Analyser's read_edf function.