Replace ComboIterator with std::set<std::vector<int>> via combinations.hpp

Since the number of generators never exceeds 6, the maximum number of combinations is (6, 3) = 20, so the space optimizations of using an iterator is mute.

Doing this way also allows to use set_difference and set_union to deal with collections of subgroups. This is not easily possible otherwise.

vis/include/geometry.hpp

@@ -5,7 +5,6 @@
 #include <optional>
 #include <numeric>
 #include <iostream>
-#include "combo_iterator.hpp"
 
 template<unsigned N>
 using Prims = Eigen::Matrix<unsigned, N, Eigen::Dynamic>;