We allow a "non-simple" prefix at top level of processes.
Examples: New k.New g.In(x). (P1|P2) is allowed if P1|P2
is a simple process.
&& Done:
      - Process.from_par_to_multiset replaces a process starting with
Par at top level into a multiset of processes and order them (i.e. it gives
them a unique ID)
      - Starting by accepting any action until we reach a Par, and then
apply Process.from_par_to_multiset. From now, compression and reduction
can be applied

&& Todo: - benchmarks
Fix compression (minor).
More (working) examples.
APTE takes new argument (-verbose) specifying what it should display.