add DataSet::getEvents(); refactor hasPeaks(); propagate changes

This introduces a new method DataSet::getEvents, which takes a set of q vectors and returns a set of corresponding detector events. DataSet::hasPeaks has been rewritten to use the new method. It should also be noted that the new implementation is significantly more accurate than the old version.

To clarify, in the old version, one could take a given q and obtain a detector event (x, y, frame) via hasPeaks, and then get the q value of the computed detector event. These two q values should agree, to within a very small tolerance. In the old implementation of hasPeaks, this was not the case: the two q values could differ by as much as 15-20%.