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.

[MartinSpiessl]

It might get a bit redundant if we start copying sections like this. I would rather have the description refactored such that we have a section for common keys like producer and then just point to that section for the different entry types.

[sim642]

Here's a radical idea: ditch the hand-written Markdown altogether and have everything in a nice JSON schema, see #61.