Use https for all submodules

.gitmodules

 [submodule "extern/clean-core"]
 	path = extern/clean-core
-	url = git@github.com:project-arcana/clean-core.git
+	url = https://github.com/project-arcana/clean-core.git
 [submodule "extern/typed-geometry"]
 	path = extern/typed-geometry
 	url = https://www.graphics.rwth-aachen.de:9000/ptrettner/typed-geometry.git
@@ -9,7 +9,7 @@
 	url = https://www.graphics.rwth-aachen.de:9000/ptrettner/ctracer.git
 [submodule "extern/task-dispatcher"]
 	path = extern/task-dispatcher
-	url = git@github.com:project-arcana/task-dispatcher.git
+	url = https://github.com/project-arcana/task-dispatcher.git
 [submodule "extern/nexus"]
 	path = extern/nexus
-	url = git@github.com:project-arcana/nexus.git
+	url = https://github.com/project-arcana/nexus.git