taa_tgba instances labelled by other objects than strings
in the same way Alexandre did for tgba_explicit.

* src/tgba/taatgba.cc, src/tgba/taatgba.hh: Split taa_tgba in two
levels: taa_tgba with no label and taa_tgba_labelled templated by
the type of the label.  Define taa_tgba_string (with the interface
of the former taa_tgba class) and taa_tgba_formula for future use
in ltl2taa.cc.
* src/tgbaalgos/ltl2taa.cc, src/tgbatest/taatgba.cc: Adjust to use
taa_tgba_string.