Make TC header-only

One header per class, except for Group and SubGroup which are coupled.

- Action
- Cosets
- Path
- Rel

example/bench.cpp

@@ -8,7 +8,7 @@
 template<class G>
 void test(const G &group) {
     auto s = std::clock();
-    auto cosets = group.solve();
+    auto cosets = group.solve({0});
     auto e = std::clock();
 
     double diff = (double) (e - s) / CLOCKS_PER_SEC;