Better resource handling in minimization.
* src/tgbatest/ltl2tgba.cc (main): Delete the minimized automaton. * src/tgbaalgos/minimize.cc (minimize): Remove the call to unregister_variable() at the end. It was both wrong (unregistering only the first variable) and useless ("delete del_a" will unregister all these variables). Use a map and a set to keep track of free BDD variable and reuse them, otherwise the algorithm would sometimes use more variables than allocated.