Faster translation of GFa.
* src/tgbaalgos/ltl2tgba_fm.cc: Add a "recurring" mode for the translation of the child of G. This generalizes Couvreur's original trick to translate GFa as (a|Prom(a))&X(GFa) without generating an intermediate GF(a)&F(a) state that would have to be merged with GF(a) latter. This implementation will also speedup formulas such a G((a U b) & (c M d)). With this patch, translating GF(p1) & GF(p2) & ... GF(p20) into a TGBA takes 57s instead of 128s on my computer. However it alsos causes some formulas to be translated into larger automata that are not immediately reduced (the simulation-reduction is needed). For instance Fa & GFa now has a different signature than GFa, so translating 'Fa & GFa' generates two states where is used to generate only one.