added worker_group; fork: default to subgroup of worker_group; removed obsolete fork_local; join/get_result: cumulative flattened exceptions; map: subgroup of worker_group;
added worker_group;
fork: default to subgroup of worker_group;
removed obsolete fork_local;
join/get_result: cumulative flattened exceptions;
map: subgroup of worker_group;

MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle