Prescribed name of witness, in order to eliminate technical problems with ambiguous names

Currently, the execution of SV-COMP searches for files named .graphml. This worked in the past, but with the new possibility to have witnesses also with ending .yml, we need a more specific name.

Thus it was agreed in the community meeting on Nov. 7, 2023 (see minutes) that the witnesses have to be named witness.graphml and witness.yml, respectively.

This MR comprises:

• the note in the rules about the names (47aa1972) and
• the fetching of both formats of witnesses in the benchmark definitions.