Improve tracing algorithm

[chris-simpson]

The tracing algorithm expects each trace to be nearly vertical, which isn't always the case. If there's a significant region where the trace is lost, it can fail to find it on the other side because it only tries to find a peak around the last position. With pinhole masks (or close sources) it's possible that another trace might be closer to this position than the correct trace.

A suggested solution is to create Trace objects which store the coordinates along a trace, probably in a deque and have a method with extrapolates the last 2 or 3 points to determine a better estimate of the trace location. This should improve the readability of the code as well, instead of juggling various lists.