bare_trace: Fix get_duration() for window use
Created by: valschneider
The previous implementation only looked at the maximum timestamp of the traces. However, if trace windows are used to analyse a subset of the full trace, get_duration() would return an erroneous value:
(times are normalized) trace = <trace built with window=[5, 10]> print trace.get_duration()
10.0
This commit adds a lower-bound lookup to fix this issue.