Compiler warnings + add workflows

[hannahbast]

Compiling the current master on Ubuntu 22.04 produces many warnings. Its' good practice to fix them, in particular, to make sure that the latest compiler version runs through without error.

I would suggest to adapt https://github.com/ad-freiburg/qlever/tree/master/.github/workflows (maybe with fewer compiler and OS versions) and add the corresponding badges (Docker build, Native build, Format check) to the GitHub README like on https://github.com/ad-freiburg/qlever . Should be fairly straightforward to do.