Performance tuning

Adressing https://github.com/chili-epfl/chilitags/issues/26, more precisely:
* [speed] disable refinement
* [speed] try the edge detection at only one (specified?) scale
* [speed] no pyramids

Moreover:
* [precision] increase the size of the window of the edge detection

turned out to be useless.

3 bonus commits (the first ones): 2 improvements on the performance test, and 1 improvement on the detection (replacing pyrDown, which used a Gaussian bluring before subsampling, by plain and simple nearest neighbor resizing).