make relevance filter work in term of a "max_relevant" option + use Vampire SOS;
"max_relevant" is more reliable than "max_relevant_per_iter";
also made sure that the option is monotone -- larger values should lead to more axioms -- which wasn't always the case before;
SOS for Vampire makes a difference of about 3% (i.e. 3% more proofs are found)

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