convert Group::solve to tc::solve function

tc/include/tc/core.hpp

@@ -8,3 +8,7 @@
 #include "util.hpp"
 #include "cosets.hpp"
 #include "group.hpp"
+
+namespace tc {
+    Cosets solve(const Group &group, const std::vector<size_t> &sub_gens);
+}

tc/include/tc/group.hpp

@@ -41,8 +41,6 @@ namespace tc {
         }
 
         [[nodiscard]] SubGroup subgroup(const std::vector<int> &gens) const;
-
-        [[nodiscard]] Cosets solve(const std::vector<int> &sub_gens = {}) const;
     };
 
     struct SubGroup : public Group {