* 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.