Introduce entry type 'location_invariant' for YAML-based witnesses.

[SvenUmbricht]

This PR introduces a new entry type for YAML-based witnesses that describes location invariants.

[SvenUmbricht]

Something that still needs to be discussed is how the producer section is currently only refering to "tools", even though an entry could be created by a human as well (true for both location- and loop invariants). The simple fix of replacing "tool" by something like "e.g. the tool" which we discussed previously does not work, because some fields (version/configuration/commandline) do not make sense if the witness was created by hand.