ENH: Simplify solving logic

This is a combination of multiple WIP commits:

- Semantic group names - G, H
- Simplify fan<>()
- Template int deduction
- Remove _recontext_gens
- Move g_gens logic into caller
- Move subgroup logic into caller
- Move recontext logic into coller
- Move _generators, _recontext_gens to anonymous namespace
- Simplify hull, tile

vis/include/geometry.hpp

@@ -8,8 +8,6 @@
 
 #include <Eigen/Eigen>
 
-#include "combo_iterator.hpp"
-
 template<unsigned N>
 using Indices = Eigen::Array<unsigned, N, Eigen::Dynamic>;