consider "locality" when assigning weights to facts

add a bonus for chained facts, since they are likely to be relevant; (especially in a Mirabelle run!) -- chained facts used to be included forcibly, then were treated as any other fact; the current approach seems more flexible

merged

add a penalty for lambda-abstractions; the penalty will kick in only when the goal contains no lambdas, in which case Sledgehammer previously totally disallowed any higher-order construct; this was too drastic; lambdas are dangerous because they rapidly lead to unsound proofs; e.g. COMBI_def COMBS_def not_Cons_self2 with explicit_apply

renaming

fiddle with relevance filter

update docs

reorganize options regarding to the relevance threshold and decay

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)

simplify more code