Bisect all intervals

Resolves #265 (closed)

Modified the bisection algorithm in Qs2Events.cpp to check for a sign change for each pair of frames (i, i+1), 0 < i < (nframes-2). This is much safer than bisecting an arbitrary number of times (previously 4, which turned out to be insufficient for some data sets).

Added a progress indicator to reflect that this algorithm is now slower.