When generating multi-packets RPs, if the RPs associated with the two
APs have no packet in common, we fall back to generating a single-packet
(averaged) RP.
This commit changes the way the APs are selected. Until now, the two
most acute angles were selected. Now, the angles are signed, i.e. we
differentiate the APs that are before the point to generate on the
trigonometric circle, and those that are after.
Some more refactoring was achieved, and some bugs fixed.
The new class Autocalibration contains the code from the former huge
function Stock::generate_reference_point(). It is refactored a bit, but
still needs work. The function Stock::generate_reference_points() stays
in the Stock class for now.