bool parallel_multiway_merge_force_sequential
setting to force all parallel_multiway_merge() calls to run sequentially
bool parallel_multiway_merge_force_parallel
setting to force parallel_multiway_merge() calls to run with parallel code
size_t parallel_multiway_merge_minimal_n
minimal number of items for switching to parallel merging
size_t parallel_multiway_merge_oversampling
default oversampling factor for parallel_multiway_merge
size_t parallel_multiway_merge_minimal_k
minimal number of sequences for switching to parallel merging