do not use a global variable to define the number of available threads
- python/spot/impl.i: Make parallel_policy implicitly contractible.
- spot/graph/graph.hh (sort_edges_srcfirst_): Pass a parallel_policy explicitly.
- spot/twa/twagraph.hh, spot/twa/twagraph.cc (merge_states): Likewise.
- spot/misc/common.cc: Remove file.
- spot/misc/common.hh (set_nthreads, get_nthreads): Remove, and replace with... (parallel_policy): ... this.
- spot/misc/Makefile.am, tests/python/mergedge.py, NEWS: Adjust.