Updates to fixate and fixateThenChoose behaviours

[cnuahs]

Some extensions to the fixate and fixateThenChoose behaviours. Specifically:

• added an initial grace period (default: 0ms) during which the subject can break fixation without penalty
• add an INFLIGHT state between FIXATING and CHOOSE in fixateThenChoose. This provides a way to detect when the eye leaves the fixation window and is useful for implementing reaction time tasks.
• added an optional timeout to the fixateThenChoose behaviour. Have to initiate a choice (i.e., enter the INFLIGHT state) within the timeout (default: Inf). This is similar to how the saccadeDuration parameter was previously being used, but saccadeDuration now specifically pertains to the INFLIGHT state, and is interpreted literally as the maximum duration to get from the fixation window (once you leave it) to the choice window.

[cnuahs]

The default behaviour of fixateThenChoose is unchanged, and fixateThenChooseDemo works as before for me. Is there something you had in mind?

[bartkrekelberg]

Just to add something that shows the inflight option.

[cnuahs]

re: isStateXXX dependent properties,

I've been using them as a convenience in neurostim function strings when my 'stimuli' are dependent on the behaviour state... e.g., changing the fixation tolerance once the eye enters the window (e.g., something like f.tolerance = '@iff(fix.isFixating,1.5,1.0))'. A common method (in the base class) rather than dependent properties would probably require changes to str2fun()?

In any event, I agree this is a pretty specific use case and, while convenient, can probably also be achieved by comparing trialTime to startTime.STATE etc., and so maybe it should be discouraged.

[bartkrekelberg]

@adammorrissirrommada this pull request is waiting for your review.