`location_invariant` YAML entry type as schema

This adapts #59 on top of #61. Since this depends on #61 being merged, the PR diff currently isn't nice, but a nicer diff can be seen here: https://github.com/sim642/sv-witnesses/compare/yaml-schema...goblint:sv-witnesses:location_invariant.

Changes

1. Add location_invariant YAML entry type.
2. Generalize loop_invariant_certificate entry type to invariant_certificate entry type, which may target both invariant entry types. This avoids unnecessary duplication.
3. Clarifies that loop_invariant entries are strictly for loops.

These changes solve https://github.com/sosy-lab/sv-witnesses/issues/60.