Update to cudawrappers 0.4.0 and fix build

This repository had cudawrappers 0.1.0 as dependency, but didn't build against it any more due to changes against a local (diverged header-only) copy of cudawrappers. To fix this, the cudawrappers submodule is updated to version 0.4.0, include directories and the Makefile are updated. Users are now required to build cudawrappers themselves, the README is updated accordingly.

Additionally, this MR adds CI. See also here.