[distance] implement more efficient box-to-box distance computation for Cartesian; need to encapsulate this in a strategy;
[distance] implementation of point-to-box distance for Cartesian with fewer arithmetic operations;
[distance] strategy for computation of Cartesian distance between two boxes (of any dimension);
[distance] add pythagoras_box_box strategy to the set of Cartesian strategy loaded by bg/strategies/strategies.hpp;
add new tag strategy_tag_distance_box_box for the pythagoras_box_box strategy;
[distance] use pythagoras_box_box strategy for computing point-to-box and box-to-box distances;