lpy: Increase window boundary for y-limit scaling

Changed the window-dependent offset that determines the set of data the y-limits scale with to a constant 0.5 seconds before the lockloss. I looked through different lpy plots and did not see any trend features that would cause this method to fail, but it could be that the offset value needs to be adjusted. Closes #67 (closed).

Lpy wide before: https://ldas-jobs.ligo-wa.caltech.edu/~lockloss/events/1239/232129/lpy_WIDE.png

Lpy wide after: https://ldas-jobs.ligo-wa.caltech.edu/~yannick.lecoeuche/events/1239/232129/lpy_WIDE.png